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Anthropological significance of the musculus pyramidalis and its variability in man.

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ANTHROPOLOGICAL SIGNIFICANCE O F T H E
MUSCULUS PYRAMIDALIS AND ITS
VARIABILITY I N MAN
.
M. F ASHLEY-MONTAGU
Department of Aiinloniy. Ilalinemann dleilical College and Bospital. Philadelphia
FOUB FIGURES
CONTENTS
Introduction ........................................................
Gross anatomy ......................................................
Historical ..........................................................
Comparative anatomy .............................
The pyramidalis in the primates .......................................
Prosimiae ......................................................
Tarsioidea ......................................................
Platyrrhinae ....................................................
Catarrhinae .....................................................
Anthropomorpha ................................................
Hylobates ..................................................
Symphalangus ..............................................
Pongo ......................................................
P an ........................................................
Gorilla
Discussion of the findings relating to the pyramidalis i n the infra-human
primates
The pyramidalis in the Hominidae
The variations in form and structure of the pyramidalis i n man ..........
Discussion ..........................................................
Summary and conclusions
..................
.....................................................
.......................................................
....................................
.............................................
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437
442
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449
450
451
451
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452
452
452
453
457
467
481
483
INTRODUCTION
I n this paper it is proposed to consider the significance of
the pyramidalis muscle and its variability in man . Its gross
and comparative anatomy will be discussed. as also its distribution among the mammals generally. and particular attention will be paid to the character of this muscle among the
infra-human primates and the living varieties of man for
'Also the Graduate School. New Pork University
.
435
.
AYgRICAN JOURNAL OF PHYSICAL A N T R E O W L W Y . VOL XXV. NO
OCTOBEE-DBCEYBLB 1989
.3
436
M. F. ASHLEY-MONTAGU
whom information relating to it is available. The principal
purpose of the present study is to discover, if possible, how
man acquired the pyramidalis muscle, what its functions may
be, and why it varies as it does.
GROSS ANATOMY
The pyramidalis muscle in man is a muscle which belongs
to the lower abdominal wall, and as its name implies it is a
relatively small pyramidal-shaped muscle which arises by
tendinous fibres from the anterior and antero-superior surfaces of the pubic bones a t the symphysis, inferior to and between the pubic tubercle and the pubic spine, immediately
craniad to the origin of the adductor femoris longus. The
pyramidalis gains insertion into the linea alba, at a point generally situated mid-way between the crest of the pubis and the
umbilicus, by its cranio-medially coursing fibres ; the craniocaudal extent of the insertion varies from a few millimeters
up to about two-thirds of the maximum breadth of the muscle.
At its origin on the pubis the pyramidalis is reinforced anteriorly by a transverse ligament consisting of tendinous fibres
derived chiefly from the aponeurosis of the external oblique.
To this pyramidal ligament the medial portion of the inguinal
ligament also contributes tendinous fibres, some of which pass
behind the pyramidalis to gain attachment to the anterior
superior pubic ligament. The pyramidalis lies immediately
anterior to the lower part of the rectus abdominis, being separated from it by its own perimysium and a generally present
loose connective tissue. Occasionally a more marked separation of the pyramidalis from the rectus is brought about by
the intervention between them of an extension of the conjoined
aponeuroses of the internal oblique and transversus abdominis
muscles. This condition has been described by Eisler ('12)
and is referred t o in Quain ('23) ; I have observed it in three
cases. Anteriorly the pyramidalis lies in intimate relation to
the posterior surface of the aponeurosis of the transversus
abdominis, frequently lying in an outpocketing of the latter,
a fact which renders it extremely easy in such cases to over-
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
437
look the muscle completely when the conjoined lamina is elevated or removed.
Occasionally the pyramidalis is encountered lying freely
upon the rectus, its fibres in some cases blending with those
of the latter muscle, and in other cases not. This intimate
association of the pyramidalis with the rectus has led to the
suggestion that it arises embryogenetically as a doubling over
of the ventral portion of the rectus (Cowper, 1694; Gegenbauer, 1895 ; Vallois, ’26). Alix (1864) on the other hand believed that the pyramidalis originated from the same muscle
mass as the external oblique. He pointed out that the direction
of the fibres of the pyramidalis do not correspond with those
of the rectus, that the rectus is not inserted into the linea alba,
and that a folding under of the antero-medial portion of the
external oblique would result in a muscle which in every respect would agree with the characters displayed by the pyramidalis as we know it. It is of interest here to note that
Vesalius (1543) appears to have regarded the pyramidalis as
a part of the rectus, referring to it as the ‘superior principiu.’
In the Second Edition 1555 of the ‘De Fabrica’ this becomes
‘superius pricipiu recti abdominis. ’ It is not, however, certain whether Vesalius intended by these terms to recognize
a muscle distinct from the rectus.
HISTORICAL
The pubic relations of the pyramidalis are of some interest.
Upon dissection it is revealed that some of the tendinous fibres
of this muscle blend with those of the suspensory ligament of
the penis. Interestingly enough, the first anatomist to refer
to the pyramidalis, namely Nocolai Massa (1536), assumed
that this muscle assisted in the erection of the penis. Vesalius
believed that it assisted in expressing urine from the penis,
‘pene urieversum eff ormans musculum.’ Modern anatomists
do not ascribe such a function to the pyramidalis. It is generally stated that the function of the pyramidalis is to act as a
tensor of the linea alba. Judging from the relations of the
muscle such an action seems probable, but upon what other
438
M. F. ASHLEY-MONTAOU
grounds the traditionally accepted function of this muscle
rests it has been impossible to discover. Fallopius (1562),
who named the pyramidales ‘musculi succenturiati’ in the belief that these muscles were auxiliaries to the external oblique
“in performing the duty of their motion,’’ and also Riolan
(1626) assumed the function of these muscles to be to assist
in compressing the bladder and in this way to promote the
voiding of urine; such a function was also ascribed to these
muscles by Rolfinck (1656) and by Santorini (1724). In 1694
Cowper rather ingeniously suggested that “When the Diaphragm has prest the Viscera, whereby the abdomen is become
Tumid, these pull the Navel downwards, by which means they
make a more adequate compression of the Bladder in the expression of Urine, than any other Muscle of this part; though
it must be confest they all contribute their assistance in that
Action.” Although this is not the usual function ascribed to
the pyramidalis by anatomists, it is probable that in tensing
the linea alba the pyramidalis assists in increasing the intraabdominal pressure and also in the more local compression
of the bladder.
The blood supply of the pyramidalis is principally derived
through a branch of the external spermatic artery, or variably
from a branch of the pubic division of the inferior epigastric
artery. Small branches from the inferior epigastric artery
may usually be traced within the substance of the muscle. The
venous drainage proceeds through the venae commitantes of
the arteries, and occasionally through a tributary of the pubic
vein of the external iliac.
The motor nerve supply may occur in a variety of ways,
being generally derived either from one of the lowermost thoracic nerves (eleventh or twelfth thoracic), the subcostal
nerve, or from the first and/or second lumbar nerves, the iliohypogastric, the ilioinguinal, and/or genitofemoral nerves.
Testut (1884) has met with a case in which the pyramidalis
was perforated by the lateral popliteal branch of the sciatic
nerve.
SIGNIFICANCE OF PPRAMIDALIS MUSCLE
439
The pyramidalis is wanting in about 16 to 18% of human
subjects, and when present exhibits a high degree of variability in form, size, and structure. By the end of the sixteenth
century the kinds of variation exhibited by the pyramidalis
were already fairly well known. Riolan (1626) refers to the
variations which characterize this muscle both in size and in
number, as does Spigel (1627) who was, incidentally, the first
anatomist to supply an excellent figure of the muscle in its
proper relations. Riolan (1649) notes that the pyramidalis is
frequently absent in man, and suggests that in view of this
fact it can hardly be considered a very necessary muscle since
" Natura nunquam deficit in rebus necessarius. " Riolan
(1626) noted that the left pyramidalis is sometimes smaller
than the right. Crooke (1650) observed that the pyramidalis
was often wanting in man and that when this was so the internal oblique extended more medially along the inguinal ligament. Unfortunateiy I encountered this observation too late
to be able to check it on the series of bodies which I dissected.
I t would be interesting to know whether it represents a correct
statement of the facts. Rolfinck (1656) also observed that the
pyramidalis is frequently wanting in man, and noted that
occasionally only one muscle was present, this, according to
Rolfinck, almost invariably being on the right side of the body.
Diemerbroek (1672) repeated these observations. Ruysch
(1704) described a case of the presence of three pyramidales,
that on one side having been duplicated. A similar description of three pyramidales has been given by Winslow (1749).
In this century variations of the pyramidalis are noted by
Verheyen (1706), Santorini (1724), Hammer (1765), Sabatier
(1781) who describes the presence of two pyramidales 011 each
side, a total of four in one body, and Lieutaud (1782).
I n the eighteenth century we make a most promising beginning with the claim of Horner (1826) to have observed
three and even four pyramidales on each side of the body.
Hallett (1848) claims to have observed two pyramidales on
each side of the body in several cases. In connection with
such observations it may be remarked here that if the ex-
440
M. F. ASHLEY-MONTAGU
amples observed represent anything more than separate
bundles of the same muscle they must be very unusual, for
they appear to have escaped the attention of most anatomists.
Drelincourt (1693)appears to have been the first comparative anatomist to have noted the absence of the pyramidalis
in a sub-human primate, in this case a catarrhine monkey.
Tyson (1699),who refers to this case, was the fist anatomist
to note the absence of the pyramidalis in an anthropoid, a
juvenile male chimpanzee.
The Problem. From the middle of the nineteenth century
the variations presented by the pyramidalis in man began to
receive increasingly more attention, until in recent years it
has been discovered that the frequency with which the pyramidalis is either present or absent in several groups of mankind
is of some anthropological interest. It was not, however, until
1926 when Vallois published his admirable study on the significance of this muscle from the comparative anatomical and
anthropological viewpoints that the subject was for the first
time placed on a really comprehensive and firm scientific
foundation. As will be seen in the course of the present paper
in order to find an answer to certain questions methods were
independently adopted precisely similar to those utilized for
the same purpose by Vallois, and in these connections practically identical conclusions were arrived at. This is the more
interesting since the present writer did not become aware of
Vallois’ work until his own observations were virtually concluded.
The present writer’s interest in the subject of the si@cance of the pyramidalis muscle and its variability in man
was generated by a statement made by Wood Jones (’29) to
the effect that the pyramidalis of man was an example of
“the retention of primitive conditions departed from in some
degree or other in the Anthropomorpha,” (p. 306), being
“usually present in Man but absent in Anthropoid Apes”
(ibid.). Testut (1884b), Poirier and Charpy ( ’12) and Loth
( ’31) had previously expressed a similar opinion. Wieder-
SIGNIFICANCE OF PYRAMIDUIS MUSCLE
441
sheim (’OS), for example, writes that the kinds of variation
exhibited by the pyramidalis
dienen als beredtes Zeugnis dafur, das der M. pyramidalis
des Menschen-und das gilt auch f iir zahlreiche Saugetiere,
wie z. B. fur die Anthropoiden-alle Charaktere eines Organs
besitzt, welches langst der Riickbildung verfallen ist. Der
Muskel erheischt aber vor allem deswegen das allergrosste
Interesse, weil er ein schlagendes Beispiel dafiir abgibt, wie
zah gewisse Gebilde selbst dann noch im Organismus haften
und fortvererbt werden, wenn sie liingst ihre spezifische Bedeutung verloren haben.
Wiedersheim goes on to point out that the pyramidalis is
very well developed in the monotremes and marsupials, and
concludes that “Zweifellos stellt der M. pyramidalis einen der
altesten Saugetiermuskeln dar, dessen Urgeschichte weit in
die promammale Zeit zuriickdatiert.”
This is quite clear: The human pyramidalis is said to be
the vestige of a muscle that is still functional in some of the
pro-mammalian relatives of the mammals such, for example,
as the monotremes and the marsupials are conceived to be.
As we shall shortly have occasion to note this is a view which
has both before and after Wiedersheim been put forward by
many anatomists.
I n order to test hypotheses of this nature concerning the
structure and variability of the pyramidalis in man the present study was undertaken. Before all else it seemed desirable
to inquire into the actual nature of the variation of this muscle
in the sub-human primates and in man; and further, to discover, if possible, what the significance of the difference, if
any, in variation might be. Since “the retention of a primitive
condition” had been invoked to explain its presence in man,
it has been necessary to inquire into the relevant comparative
anatomy of the subject in order to be quite clear upon the
question of the phyletic status and relationships of the pyramidalis in tlie liiglier primates. In addition, the frequency with
which tlie muscle is either present or wanting in the varieties
of mail has been studied with a view to determining the anthro-
442
M. F. ASHLEY-MONTAOU
pological significance of this variability as it exists between
the several varieties investigated. In the prosecution of these
tasks the studies of many previous investigators have been
laid under toll in order to bring all the data bearing upon the
subject together, and to draw from it such conclusions which,
being based upon so much data, would be, at least quaiititatively, as reliable as possible. The report of my own observations, which were carried out over a period of 6 years from
1932 to 1938, is based upon the examination of 226 human
bodies, four female Tarsiers, one loris (Loris tardigradus),
and on one purple-faced leaf monkey (Kasi vetulus nestor).
All the human bodies were dissecting room subjects, and were
examined in the laboratory of the Department of Anatomy,
New York University. Additional data relating to this material will be found in the tables and in the body of the paper.
COMPARATIVE ANATOMY
It is frequently the case in comparative anatomical studies
that structures in different orders of animals bearing a superficial resemblance to one another are taken to be homologous,
which upon further detailed study are found to be only analogous. The pyramidalis is a case in point. The earlier anatomists not being under the necessity of pressing their facts into
the service of a particular theory were clear of any confusion
here. Thus Tyson (1698) who was the first anatomist to give a
thorough description of the anatomy of a marsupial, a female
South American opossum, carefully described the marsupial
bones and the muscles attached to their ‘internal side,’ and
named them from their shape the Triangularis, thus distinguishing them from the pyramidalis of man with which he was
intimately familiar from his many dissections. Cuvier (1846)
recognized that the triangulares of marsupials were ‘analogues
aux pyramidaux’ and not their homologues. Briefly, the triangularis of marsupials arises by a broad attachment at the
symphysis and from the anterior and inner borders of the
marsupial bones and meets the opposite muscle in a tendinous
raphe a t the aperture of the pouch. Superiorly these muscles
SIGXIFICANCE O F PYRAMIDALIS MUSCLE
443
are attached to the xiphoid process. Between the muscles in
the region of the marsupium lie the mammae, so that when
the triangulares contract they approximate these bones toward
one another, and thus serve to compress the glands and to
express the milk which they contain through the orifices of
the nipples. According to Tysoii (1698), Saint-Hilaire (1823),
Meckel (1826), Cuvier (1846), Alix (1867 a), Coues (1871),
Testut (1884 b), and Vallois (’26),they also assist in opening
and closing the pouch. It is difficult to detect any but the most
superficial resemblance between the marsupial triangularis
and the hominid pyramidalis. The form, relations, and functions of the marsupial triangularis and the pyramidalis of
those mammals which possess such a muscle are quite different, yet in spite of these obvious and significantly important
differences most comparative anatomists who have discussed
this matter have taken it for granted that the highly specialized ‘ pyramidal’ of the marsupials was homologous
with that of man. This was the view expressed by Testut
(1884), Gegenbauer (1895), Le Double (1897), Wiedersheim
( ’OS), Hopf ( ’09), Tschachmachtschaii ( ’12), and in recent
years by Wagenseil (’27), Loth (’31), Neal and Rand
(’36), and Howells (’39). Testut (1884), for example, who
was quite aware of the highly specialized nature of the marsupial pyramidal, nonetheless regarded this muscle as presenting its ‘typical’ form in this order, and wrote that
wherever else these muscles were found among the bionodelphia, particularly in man, they merely represented vestiges
“proclaiming their existence as part of the general plan of
mammalian organization, but otherwise absolutely useless.
Hence, the variations in form, size, and number encountered
in man, all evidences of the tendency towards atrophy and
ultimate complete disappearance.” The fact appears to be
that Testut himself never dissected the ‘pyramidal’ in a marsupial, and moreover seems to have been quite unacquainted
with the frequencies with which the pyramidalis varies in
size, form, and number in man. Testut’s confusion was probably originally due to his adherence to the belief in the marA I B R l C A N J O U R N A L O? PHYBICAL ANTHPOPOII)OY, VOL. XSV, NO.
3
444
M. F. ASHLEY-MONTAGU
supial origin of the placental mammals. The type of argument
generally proceeds upon the following lines : If the placental
mammals originated from the marsupials, and if the marsupials are characterized by enormously developed pyramidals
(which may stretch the whole length of the abdomen), then
all mammals possessing a pyramidalis must have inherited it
from a marsupial ancestor. Furthermore, if the placental
mammals more often than not exhibit complete absence of this
muscle or, as in man, display a comparatively small muscle
which is sometimes wanting on one or both sides of the body,
occasionally multiplied, and often altogether suppressed, then
it is clear that this variability of the muscle in all such mammals indicates that the pyramidalis is in process of disappearing altogether ; its occasional ‘vestigial’ presence being best
regarded as the retention, though in much attenuated form,
of a primitive condition. The most recent proponent of this
view is Loth ( ’31).
The truth is that today few, if any, comparative anatomists
could be found who would subscribe to the theory of the metatherian origin of the Eutheria. On the other hand the evidence
now fairly conclusively indicates that both the marsupials and
the monotremes are to be regarded as early offshoots from
the primitive stock common both to them and to the mammals.
Neither stand in the direct line of eutherian evolution hence
it cannot reasonably be argued that the eutherian pyramidalis
represents the ‘vestigial’ form of the metatherian pyramidalis. The argumentum ad vestigium merely begs the question
(Ashley-Montagu, ’38). To infer, as some have done, a prototherian ancestry for the eutherian pyramidalis is, it is to be
feared, based upon a misunderstanding. As Bensley (’03)
points out
the Marsupials and Placentals are collateral and, in a certain
sense, equivalent groups, of common parentage ; and this conception may be welcomed as clearing the way for a better
perception of the details of their secondary evolution or adaptive radiation. Especially is this true of the former: any
attempt to explain their secondary differentiation on the basis
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
443
of a Marsupial ancestry of the Placentals must naturally result in confusion, because of the lack of distinction between
those of their adaptive characters which are, generally speaking common to all of the members of the group and those
distinctive of minor divisions, leading to the doubly erroneous
conclusion that the evolution of the Marsupial group is not
finite, and that characters of both kinds have been carried over
from Marsupial to Placental stages.
Matthew (’04, ’15, ’16, ’28) has expressed very similar
views.
Vallois (’26) has shown that a muscle resembling the pyramidalis is to be found in such different vertebrates as the
Caudata (Cryptobranchus, Salamandra, Molge, Axolotl, etc.),
Crocodilia, Squamata (lizards and snakes), Prototheria (Ornithorynchus and Echidna), and Metatheria (marsupials),
and he has further shown that this muscle cannot be regarded
as the homologue of the eutherian pyramidalis since its structure and relations differ in a fundamental manner from those
existing among the latter. He writes:
L’Qtude des Amphibiens et des Reptiles nous a montre qu’il
n’existe chez ces Vertbbr6s aucun muscle comparable au pyramidal des MammifBres. Les diverses formations que l’on
a designees sous le nom de pyramidal sont, en effet, tr6s differentes non seulement du pyramidal mammalien, mais m6me
entre elles : le soi-disant pyramidal des Crocodiliens n’est que
la partie caudale, post-pubienne, du droit superficiel; le pyramidal des UrodBles est la portion pr6-pubienne des fasceaux
profonds du droit superficiel; le pyramidal du Sphenodon et
des Autosauriens est le droit profond, plus ou moins reduit
en volume.
Entre ce divers muscles et le pyramidal des MammifBres,
qui est un muscle pr6-pubien, situ6 ventralement a u droit, il y
a une certaine analogie dans la direction des fascieaux, mais
il n’y a aucune homologie r6elle. La senle chose que nous
montrent les non-MammifBres, c’est que, dans les deux
groupes des Batraciens et des Reptiles, certaines des fibres de
la masse longitudinale ventrale du tronc peuvent devenir plus
ou moins oblique, et, de ce chef, se terminent sur la ligne
m a i a n e ; mais, toujours, ce sont des fibres profondes qui se
comportent ainsi.
446
M. F. ASHLEY-MONTAQU
Une premiere conclusion se d6gage done: le pyramidal des
Mammif&res n'a pas d'homologue chez les autres VertCbrCs.
C'est une n6o-formation propre it cette classe.
It would hardly seem possible hereafter for anyone to suggest with any seriousness the existence of a phylogenetic
relationship between the metatherian pyramidal and the eutherian pyramidalis. When we inquire into the conditions
among the most primitive living representatives of the
Eutheria, namely the Insectivores, it is found that the pyramidalis is generally present, thus Leche (1883) found a welldeveloped pyramidalis in the following species : Myogale,
Tupaia, Erinaceus, Talpa, Sorex, and Centetes. I n Gymnura
it was rudimentary, and in Macroscelides absent.* Dobson
(1882) found the pyramidalis present in Chrysochloris, Nicrogale, and Centetes. Vallois ('26) found the muscle present in
Erinaceus europaeus, Talpa europae, Myogale pyrenaica, and
in Centetes setosus. Le Gros Clark ('26) failed to find any
evidences of the pyramidalis in four adult male pen-tailed
tree-shrews (Ptilocercus lowii).
The fact that the pyramidalis is so generally found among the
insectivores is of very great interest, since, in the first place,
it is generally recognized that an insectivore-like creature was
very probably ancestral to all the radiating branches of mammals (Matthews, '04, '28), and in the second place, among the
genera of insectivores there is one that makes an ideal generalized ancestor for the primates, namely Tupaia-and Tupaia
possesses a pyramidalis. Here then is a possible ancestor for
the primate pyramidalis. There are certain facts, however,
which at first render it difficult to accept such a suggestion.
If some insectivore-like creature is to be regarded as ancestral
to the primates, and if on the basis of the fact that the insectivores as we know them today, with the exception of Gymnura
and Macroscelides, possess a well-developed pyramidalis we
should not unreasonably expect to find this muscle present in
* "Bei allen untersucbten Inseetivoren, ausser bei Macroseelides und Gymnura,
bei welchen er fehlt (Macroacelidea) oder rudimentiir ie (Gymnura), kommt dieeer
Muskel im wohl entwiekelten Zustnnde vor." Leche (1883), page 62.
SIGNIFICANCE O F PYRAMIDALIS MUSCLE
447
maiij- representative mammals. But as Chudzinski (1898) long
ago noted the pyramidalis occurs only very exceptionally
ainoiig the mammals. Thus, it is absent in the Edentata,
Roclentia, Carnivora, Cetacea, and Ungulata. I n the Cheiroptera the muscle is occasionally present, and it has been found
in certain Microcheiroptera by Alix (1867 b), Humphry
(18'72), Macalister (1875), Jlaissoiineuve (1878) and Vallois
( '26). The muscle appears to be practically always present
in the species of the genus Pteropus, but according to Vallois
( '26) is as frequently wanting in Verspetilio and Rhinophus.
According to Leclie (1874) the pyramidalis is wanting in the
Sirenia.
Not all of the approximately 10,000 species of these seven
orders of mammals have been investigated and reported upon
with particular reference to the pyramidalis, so that it is quite
possible that in some species this muscle may be present.
However this may be, the fact is that on the basis of our available knowledge it is possible to say that among the members
of the orders named the pyramidalis is usually not present.
If all the members of the Cheiroptera have a common origin,
and this is generally agreed, then some of them have either
( a ) lost a muscle which was ancestrally present, or (b) inherited a condition in which the muscle was ancestrally not
present. From alternative (a) it would then follow that some
others had retained the ancestral condition, and from alternative (b) that in still others (i), a very primitive condition
(with modifications) had reappeared, or (ii), a n altogether
new structure had come into being. But such logical exercises
can offer no definite solution. When, however, we recall that
in the insectivore Gymnura the pyramidalis is rudimentary,
and in Macroscelides completely wanting, a solution of the
problem suggests itself. Gymnura happens to be the one genus
among the mammals which in its generalized morphologic
characters stands nearest to the assumed form of the common
ancestor of all the mammals. If the pyramidalis of the latter
was anything like that of Gymnura then it may readily be
understood that its disappearance in some and its develop-
448
M. F. ASHLEY-MONTAOU
ment in other grotips of mammals could easily be accomplished. Hence it is not difficult to understand how it is that
in such an highly specialized order a s the Cheiroptera the
muscle may have suffered varying fates in different species.
THE PYRAMIDALIS IN THE PRIMATES
When we turn to inquire into the present distribution of
the pyramidalis among the primates the facts are found to
be of somewhat more revealing interest than those relating to
the conditions among the non-primate mammals.
Prosimiae
Among the Prosimiae the pyramidalis has never been observed to occur, a t least not among any of the various genera
and species which have thus f a r been investigated by Rfurie
and Mivart (1865), Leche (1874), Milne-Edwards and Grandidier (1890), Tschachmachtschan ( '12), and Vallois ( '25), and
among which are Lemur rufus, macaco, catta, nigrifons and
Loris tardigradus ; nor was it present in a mature female dissected by the present investigator; nor in Chiromys (Murie
and Mivart, 1865) ; Galago garnetti (Murie and Mivart, 1865 ;
Tschachmachtschan, '12) ; Perodicticus, Indris, Avahi, arid
Propithecus (Milne-Edwards and Grandidier, 1890 ; Loth, '31).
Tarsioidea
The pyramidalis in the Tarsiers is a difficult muscle to display in a n ordinary dissection since it is not strikingly differentiated from the rectus, and hence requires careful
dissection under the binocular microscope if it is not to escape
the investigator's eye. Under such conditions the muscle when
present is seen to be quite distinctively differentiated from
the rectus presenting, in all its characters, features which a r e
in no way distinguishable from those characterizing this muscle in the higher primates. The belly of the muscle is about
12 mm. long in the adult Tarsier, rather flat and delicate.
Burmeister (1846) apparently failed to find this muscle in the
SIGNIFICANCE OF PYFUMIDALIS MUSCLE
449
specimens which he dissected. Alix (1865) found the muscle
present in one or more specimens (Tarsius spectrum Geoff.)
which he dissected, its belly measuring more than 1 cm. in
height. I n view of the fact that there appears to be some
doubt concerning the character of the pyramidalis in Tarsius
the classical description of Alix may be quoted here. Writing
of the rectus he says
il s’attache a u pubis dans tout l’espace qui shpare la symphyse
de 1’6minence il8o-pectin6e. I1 est recouvert, pr6s de cette
insertion, d a m une hauteur de plus de 1 centimhtre, par un
plan charnu iL fibres obliques qui n’est pas autre chose que le
muscle pyramidal, remarquable ici par son dhveloppement et
attach6 au pubis dans une 6gale 6tendue.
Woollard ( ’25) found a quasi-pyramidalis present in Tarsius
spectrum, writing “It is true that the pyramidalis cannot be
separated as a distinct muscle, but there can be no doubt that
it is included in the broad origin of the rectus and fibres from
t,he lateral portion can be traced into the linea alba” (p. 1159).
Whatever the conditions may have been among the specimens examined by Woollard it is clear that Alix was impressed with the remarkable development of the pyramidalis
and its clear differentiation from the rectus. The present
writer dissected four female mature Tarsiers from Mindanao
in the Philippine Islands, and found the pyramidalis clearly
and distinctly differentiated as a separate muscle lying upon
the rectus in two cases. I n the other two cases a pyramidalis
was not demonstrable, although in these two cases it is not
possible to be completely certain that the pyramidales were
wanting since the abdomen had been opened u p before being
sent to me, and dissection of the critical region had thus been
rendered difficult. From the available data it would appear
reasonable to infer that the pyramidalis is generally present
and sometimes wanting in the Tarsiers.
Platgrrhinae
Among the Platyrrhinae the pyramidalis is known to occur,
but in this group of primates the muscle is apparently subject
to much variation. Loth ( ’21) found the pyramidalis present
450
M. F. ASHLEY-MONTAGU
in an Hapale penicillata and in a Midas leonina. Tschachmachtschan ('12) failed to find the pyramidalis in the three examples of Hapale jacchus which he dissected, and Beattie ( '27)
failed to find any evidences of it in fourteen other representatives of this species. Tschachmachtscha.n failed to find any evidences of the muscle in two Cebus flavus and two Ateles ater.
Icohlbrugge (1897) and Leche (1874) have each described an
Ateles in which the pyramidalis was present, and Testut
(1884) and Meckel (1528) found it present in Callithrix. In
eight platyrrhines, which he leaves unspecified, Loth ( '31)
failed to h d a pyramidalis.
These findings for the thirty-five platyrrhine monkeys thus
far reported yield a frequency of six cases in which the pyramidalis was present, or a total of 17.1% of occurrences.
Catarrhinae
Among the catarrhiiie monkeys the pyramidalis is very
variable in its occurrence among the various groups represented. Meckel (1828) found the muscle present in a Papio,
and Vallois ('26) found it in another, as did Loth ('31) in n
P. hamadryas, but Cliampneys (1872) failed to find the pyramidalis in a P. anubis, as likewise did Michaelis ('03) in a
P. babuin. Out of five Cercopitheques (Cercopithecus campbelli ? , C. maxwelli ? , C. pygerthrus 6, C. cynosurus 0 , and
C. patas 8 ) Tschachmachtschan ( '12) found the muscle present only in a male C. pygerthrus. Leche (1874) found the
muscle present in a C. cynosurus, as did Loth ('21), and
Meckel (1828) found it present in a C. sphinx. Vallois ('26)
found the muscle present in another cercopitheque, though
poorly developed. Meckel (1828) found the muscle present in
a Semnopithecus sabae, and Tschachmachtschan ('12) out of
t,wo female and one male Semnopithecus cephalopterus found
the muscle present in oiic animal but fails to mention its sex.
Kohlbrugge (1897) found the muscle present in an unstated
number of semnopitheques dissected by him. The present
writer has found the muscle extremely well developed, but
present on the right side oiily, in an adult female purple-faced
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
451
leaf monkey (Kasi vetulus nestor, Ceyldn). I n this case the
form and relations of the muscle were essentially similar to
those encountered in man. Out of five Macacus cynomolgus
Tschachmachtschan ( ’12) found the pyramidalis present in
two out of two males and in one out of three females. I n six
macaques Vallois (’26) found the muscle present in four cases.
In all the groups of catarrhine monkeys thus far examined
the pyramidalis was found to be more frequently present than
absent, and out of the total of thirty here listed eighteen animals, or SO%, showed the presence of the muscle (in one case
on the right side only). Yet as late as 1931 Loth (’31)could
write “La conclusion est que chez les Catarrhiniens l’absence
du muscle est la r6gle.”
Anthropomorpha
Hylobat es
The pyramidalis was present in the single examples of
Hylobates agilis 6 , and H. leuciscus 6, dissected by Kohlbrugge (1890), who writes that “bei H. leuciscus war an
der rechten Seite kein M. pyramidalis vorhanden, an der
linken Seite fand ich zwei, von denen der eine in der Medianlinie, der andere lateral von derselben entsprang ; beide
endeten in der scheide des linken M. rectus abdominis.” This
animal represents the only infra-human primate in which such
a variation has been noted. The muscle was wanting on the
side examined in a Hylobates lar 6 dissected by Stewart (’36).
Symphalangus
The pyramidalis was wanting in the example of Symphalangus syndactylus 8 dissected by Kohlbrugge (1890), and in the
two examples dissected by Tschachmachtschan (’12) ; it was
present, however, in two out of seven animals examined by
Loth (’31), and in the animal reported upon by Deniker
(1885).
Thus, out of the fourteen representatives of the Hylobatidae
examined by various investigators the pyramidalis was present in five cases, or in 35.776 of the cases thus far investigated.
452
M. F. ASHLEY-MOXTAQU
Pongo
The pyramidalis has not been found to occur in any of the
orangs thus f a r investigated by Bischoff (1879), Fick (1895 a,
1895 b), Michaelis ( '03),Loth ( '21), and Stewart ( '36), a total
of nineteen specimens. We may conclude that the pyramidalis
is probably always wanting in the orang, although it is possible
that it may occasionally occur.
Paiz
According to R6zycki ( '22) the pyramidalis is nearly always
present in the chimpanzee. R6zycki was apparently led to
make this statement on the basis of the fact that the muscle
was present in five out of the six juvenile chimpanzees examined by him. Gratiolet and Alix (1866) found the pyramidalis
present in the specimen examined by them (Troglodytes
Aubryi), describing it as 'assez d6velopp6s.' It was present
on both sides of the body in the specimen dissected by Sperino
(1897), and Vallois ( '26) also found the muscle well developed
in a female chimpanzee. It was found present on the left side
only in a juvenile male chimpanzee dissected by Stewart ( '36).
Tyson (1699) in a male, Trail1 (1817) in a female, Champneys
(1872) in a female, Duvernoy (1855), Vrolik (1841), and
Michaelis ('03) in a female, and probably Bischoff (1879)
found the pyramidalis to be wanting in the single examples
examined by them.
Among the chimpanzees here brought together, a total of
seventeen cases, ten show the presence of the pyramidalis and
seven show its absence, thus 58.8% of chimpanzees thus far
investigated show the presence of the pyramidalis.
Gorilla
The species of gorilla thus far examined and reported upon
in connection with the pyramidalis is the lowland species
Gorilla gorilla. The pyramidalis was wanting in the specimens examined by Sommer ( '07)' Bischoff (1879), Loth ( '21),
and on the single sides examined in their specimens by Stewart
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
453
( '36) and Raven,' and in one out of the two examples studied
by Deniker (1885), being present in a foetus ; the pyramidalis
was present in an adult gorilla dissected by Duvernoy (1855)
and wanting in a juvenile gorilla dissected by the same investigator.
Thus in three out of the nine gorillas for whom observations
are available, or in 33.3% of cases, the pyramidalis was
present.
Among the anthropoids then the pyramidalis occurs in
35.7% of gibbons, in 58.8% of chimpanzees, in 33.3% of
gorillas, and is generally if not always wanting in the orang.
DISCUSSION OF THE FINDINGS RELATING TO THE PYRAMIDALIS
I N THE INFRA-HUMAN PRIMATES
Among the infra-human primates we observe that the pyramidalis occurs in many genera of the Platyrrhinae (17.1%
of all cases), and Catarrhinae (60% of all cases), and in the
anthropoids with the exception of the orangs (gibbon 35.776,
chimpanzee 58.8%, and gorilla 33.3%). The pyramidalis does
not occur in the Prosimiae, but it is present in a majority of
Tarsiers. Among mast mammals, we have already noted, the
pyramidalis appears relatively rarely, being limited to but a
few groups. The true pyramidalis would appear to be an
insectivore-primate characteristic. The so-called pyramidalis
of the lower vertebrates, as Vallois ('26) has shown, cannot
reasonably be regarded as the homologue of the mammalian
pyramidalis, and hence must be excluded from any discussion
of possible relationships. Both in the insectivores and in the
primates the pyramidalis is variable in form and in the frequency with which it appears. In both Orders it is apparently
entirely wanting in some families and rudimentary in others.
When all the evidence is finally available such peculiar differences in the distribution of this muscle will probably be found
to represent the expression merely of the combined effects of
ecological and genetic factors. What seems fairly evident now
is that the ancestry of the primate pyramidalis is most proba' I am able to quote this case through the kindness of Mr. Harry Raven of the
American Museum of Natural History, New York.
TABLE 1
RACE 09 OROUP
d
9
No. % No. %
d9
~~
--
- --.
-
__
American Whiti Cambridgqhfaea.
American Whiti Cambridgqlaea.
American Whitc Denver, Colorado
American WhitC Chicago,Illinoia
American white
American Whiti
._
d?
d
9
(337) (225) (112)
250
165
85
81
16
97
143 130
13
191
168
23
--.
681 544 137
Dwight, 1895
Dwight, 1895
Chouk6, '35
Amon et al., '38
No.
67
25
36
37
165
26.9 40
26.0 24
25.2 35
18.8 31
24.2 130
24.2
29.6
27.0
18.5
24.0
27 31.7
1 6.2
1
7.7
6 26.1
35 25.6
95 26.0 55 25.6 40 27.0
? 1
1
26124.71 ?
365
105
216
1
149
?
Scottish .
Scottish
95
50
1
?
1
61 6.31 1
10 20.0 ?
1
1
1
?
?
1
?
Irish
60
1
1
14 23.3
?
1
?
1
177
243
98
59
577
128
107
54
32
321
49
136
44
27
256
27 15.3
26 10.8
21 16.4 6
9 8.4 17
Czeckanowski, '06
Loth, '21
h b l a n c et al., '30
Denver, Colorado Chouk6, '35
138
200
59
12
93
120
f
11
45
80
18 13.0
1
?
13 22.0
3 25.0
Negro
Negro
Negro
Negro
Negro
Negro
Varioua
3. W. Africa?
Denver, Colorado
3hicag0, Illinois
New York, N. Y.
Loth et al., '12
Streblow, '15
Chouk6, '35
Anson et al., '38
Ashley-Montagu, '38
(53)
4
13
72
34
123
?
1
11
65
30
107
?
3
2
7
4
16
1
4
11
3
19
Japanese
Japanese
Japanese
Japanese
Japanese
Japanese
Ikayama
Adachi, 1900/10
Adachi, 1900/10
Koganei et al., '03
Hibi, '31
Nakamura, '35
85
72
96
20
60
333
65
52
70
16?
f
203
20
20
26
41
1
70
4
3
31
1
2
13
4.7
4
4.2
2
3.11 21
5.0
1
3.3 ?
4.0
9
-
- _ _ _ _
English
Scottish
French
French
French
French
French
SWiW
Polish
Berbers
Mexican
Zurich
Warsaw
Kyoto
rokyo
Kokuriku
Kyushu
Chinese
Chinese
3
20
17
-.
40
Chinese
Chinese
Totals and averaeei :
454
2
19
14
35
?
1
1
1
3
5
12.2
12.5
t
l
l
I
t
?
t
1
9
1
1
1
t
l
f
P
f
t
1
25.0
30.7
15.3
8.9
15.4
T
7 7.5 11 244
?
1
1
!
?
? ?
1
1 9.1 1 100.0
9
1
1 33.3
2 18.2 2 100.0
10 15.4 1 14.3
3 10.0
15 14.0 4 25.0
-
1
-
-
6.1 - 4.0 1 5.0
2.9? 11 3.9
6.2
1 1
4.4 2 2.9
-
- _ - - -
15.9 ..
12.5 -
- - __
- - -
TABLE 1
d9
d
0
No. % No. % No. %
47
20
22
36
25
18.8
20.6
15.4
18.8
18.3
25
19
21
30
95
15.1
23.5
16.1
18.0
17.3
22 25.9
1 6.2
1 7.7
6 26.1
30 22.C
67 18.3 37 17.1 30 20.a
? ?
24122.7) ? ?
61 6.3? ?
3 6.0?
T
?
I
?
?
?
?
?
?
?
9 15.0
JTAL NUMBEE or n m g IN WHIOE
THl YUboLB WAS WAXTIIO
Left side only
Bath l l d u
No.
d9
8
9
No. % No. % No. %
% No. % No. %
5
4
8
1
2.0
4.1
5.6
0.5
18 2.6
5
4
8
1
38
4.0
5.0
6.1
0.6
3.3
------
15 6.0 10 8.0
1 1.0 1 1.2
6 4.2 6 4.6
22 3.2 17 3.1
5 5.8
- -
-5
-
9 2.5 7 3.2 2 1.3
19 5.2 11 5.1 8 5.:
'robably 2 w e s absent on one (undated) aide.
Sex unstated
?
? ?
? ?
? 1 T ? ? ? ?
T
n 7 cam8 absent on one (unstated) side.
Sex unstated
35.01 T T
?
23.3?
? ?
?
21 11.9 16 12.5 5 10.:
17 7.0 6 5.6 11 8.1
?
?
T t
? t
?
?
1 1
1 t
?
t
? I ? ?
4 2.2 3 2.3 1 2.0
3 1.2 1 0.9 2 1.5
T
?
?
?
?
?
2 1.1 2 1.6 - 6 2.4 2 1.9 4 3.0
? ? ? T ?
?
(
T
T
?
?
?
?
?
9 6.5 3 6.6 6 27.7
? ? I ? ? ?
1
?
?
?
?
9
1
8.3
1
?
T
7
?
T
?
t
? ?
9.1 - -.
?
? ?
T ?
1 25.0 - - 1 33.:
2 15.4 1 9.1 1 50.(
7 9.7 7 10.8 - 2 5.0 2 6.6
12 9.7 10 9.3 2 12.!
?
--
2
3
3
1
9
2.3
4.7
3.1
5.0
2.7
2
2
21
1
?
?
T
?
?
?
?
T
?
?-
?
l
?
2 16.6
1 9.1
1 -
?
T
?
T
T
T
T
l
T
?
?
9 6.5 4 8.3 5 21.7
? T ? ?
?
?
l
?
?
?
T
?
?
-----2 15.4
2 2.8
--
1 9.1 1 50.C
1 1.5 1 14.3
2 2.8
2 3.1
-
-
- - - - ~1 2.9 1 3.3 - -
4 3.2 2 1.9 2 12.:
3 2.5 3 2.8
-
3.1
4.0 1 5.1
2.91 l? 3.1
6.2 - -
- -
L41
114
45
58
73
L31
46
No. %
20.9 81 18.0
22.8 65 19.7
23.2 43 22.2
20.3 56 21.5
19.1 61 18.1
21.1 306 20.0
162 22.2
------- -- -1 5.9 I 7.1
1 2.5 1 2.9
---
--
60
49
2
2
12
125
27.0
29.0
6.2
7.6
26.1
25.1
70 23.5
50?23.4
24T12.6
T
?
?
T
?
?
?
1
13 13.0
23 19.2
?
?
I
I
?
?
?
?
11
28
21
9
69
13.5
8.6
27.2
17.7
14.3
37
15
32
12
96
36
82
26
4
13.0
20.5
22.0
17.0
14 7.5 22 24.4
67 28.0 15 9.4
11 10.5
2 25.0
6 23.1
18 12.5
5 7.3
4.2 16.6
?
14.4
7.0
30.0
18.5
15.0
11.3
10.3
23.8
16.6
13.4
48
43
53
21
165
?
?
?
3 75.0
3 13.6
?
1
3 13.6 3
17 13.1 1
5__-8.3 25 11.7 5
?
?
- -
4.6
3.8
2.9
6.2
2.9
1.2
-
15.6
2 5.0
2 3.8
-
?
?
4.0
7
4 2.9
I 3.6 1 1.4
?
33.3
75.0
7.1
- -
_ - - - - 1
1
455
No. 46
92 21.3
6 3.5
6
6 4.1
4
6 3.1
4
2 5.0
2
4 3 . 3 ?
24 3.6 16
7 3.4 2 2:
9
d
No.
-
-
456
M. F. ASHLEY-MONTAOU
bly insectivore. Why the Lemuroidea or any other members of
the Prosimiae do not possess a pyramidalis it is difficult to say,
unless an appeal be made to the genetic argument. The Tarsioidea do, on the other hand, generally possess this muscle, but
even in this primitive family we already find it sometimes lacking. Variability of the pyramidalis may, therefore, be regarded
as a primitive attribute of that muscle. Upon the basis of the
evidence it would appear most reasonable to look for the
ancestor of the human pyramidalis among the Catarrhinae.
I n this group, as we have seen,‘the pyramidalis is found frequently and well distributed (60%). The stock which presumably gave rise to the African apes and to the Hominidae will
never, it is to be feared, yield up its soft parts for examination,
nonetheless it appears more than likely that the ancestors of
the African apes and man inherited a pyramidalis from some
lower catarrhine ancestor who was already characterized by
a marked genetic variability, and that this quality or these
qualities of variability was transmitted in varying degrees to
each of its descendant lines. The subsequent genetic history
of such descending lines may readily account for the fact that
the pyramidalis is more frequently present in the chimpanzee,
for example, than in the gorilla. It would seem well nigh
impossible upon any other hypothesis to account for the frequency differences which, as we shall see, exist in the distribution of the pyramidalis among the races of mankind.
We may then conclude the first part of our inquiry by resting upon the facts, facts which unequivocally prove the unsoundness of such statements as that the pyramidalis in man
is an example of “the retention of primitive conditions
departed from in some degree or other in the Anthropomorpha,” and that the muscle is usually “absent in the Anthropoid Apes’’ (Wood Jones, ’29). As we have already seen the
pyramidalis occurs in all the anthropoid apes with the possible
exception of the orang. We have also seen that it is present
in many of the platyrrhine and catarrhine monkeys. I n fact,
the pyramidalis may almost be regarded as a primate monopoly among the mammals. I n view of the facts, therefore, we
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
457
cannot reasonably infer any other than a catarrhine origin
for the hominid pyramidalis. As we shall later see, far from
representing the retention of a primitive character which, as
some investigators believe, is in process of disappearing, the
pyramidalis shows every sign of becoming established as a
relatively constant feature of the human abdominal wall.
As for the alleged ‘homology’ of the hominid pyramidalis
with that of the marsupial and monotreme pyramidal, we have
seen that there is here no true homology and no significant or
necessary relation between the two structures.
THE PYRAMIDALIS I N THE HOMINIDAE
Before proceeding to the consideration of the significance
of the variability of the pyramidalis in man it is desirable to
inquire into the frequency with which this muscle is wanting
in the various human groups for which it has been recorded.
The available data has been analyzed and is presented in
table 1. Unfortunately that table presents many lacunae owing
to the extremely unhappy methods which certain investigators
have adopted in presenting their data. For some reason best
known to themselves most investigators have chosen to present their data as a percentage of presence or absence of the
pyramidalis in the total number of sides examined. In such
cases sex is frequently not given, nor are the sides distinguished as either right o r left or both sides, and as a consequence it is generally quite impossible to discover precisely
in what manner the absence of the muscle actually manifested
itself. Wherever possible the essential data has been calculated from the figures provided by each investigator, and in
some cases this has resulted in the correction of some obvious
arithmetical errors. Where there has been any serious doubt
concerning any investigator’s figures these have been entirely
omitted.
I n four cases (Drelincourt, 1693; Hammer, 1765 ; Hibi, ’31;
and Nakamura, ’35) it has been impossible to consult the original work of each investigator, in the first three cases because
they were simply not procurable, and in the fourth case be-
TABLE a
_-_
--
-
._
TOTAL I W Y B - OI
WBJXOTE -MINEC
R A C E OR OBOUP
INVSBTIOATOB
lroCALITY
-.._
__.
-
Dwight, 1895
Chouk6, '35
Anson et al., '38
~
~
JP
.__.I
$9
d
250
97
165 85
8 1 16
9
---
.
TOTAL KUYBEO 0. SUBJDCFO Iw
N E I O H TIi6 M U 8 C U W M
WANTINQ O N O N S 09 BOTS w a o
No.-
67
25
26
37
.
27 31.7
1 6.2
1 7.7
6 26.1
26.9
26.0
25.2
18.8
Cambridge
Denver
Chicago
English
Great Britain Thomson et al., 1895
365
216 149
95 26.0
French
Tours
Le Double, 1897
243
107 136
26 10.8
Alsatian
Stramburg
Schwalbe & Pfitener, 1894
177
128
27 15.3
Swisa
White
Mexican
Zurich
~-
40
24
35
31
NewYork
Okayama
Kyoto
/Ashley-Montsgu, '38
Tsiain and averages : .
j
1
... .. .
55 25.5
9
30
4
17
21 16.3
65
20
96
70
26
21 16.4
Cliineae
Chinese
Chinese
Chinese
Mongol
Homo saniena
s -
6
12.2
__
-
.
1 9.1
1 100.0
1 20.0
1 100.0
1 33.3
2 100.0
1 14.3
- -
2 18.2
10 15.4
3 10.0
- -
16 14.3
.
-
4 4.7
4
3 4.2
2
3 3.1
2
1 5.0
1
2 3 . 3 7
13 '4.0
-
27.0
l
i 12.5
8.4
_..
129 112
85
%
7.5 -11. 24.4
-.
331 20.7 222 20.5 109 21.1
I-
Adachi, 'OO/ '10
Adachi, 'OO/ '10
40
18 13.0 - - . 7
-
2 33.3
1 25.0
4 30.7
1 1 15.3
3 8.9
34
24.2
29.6
27.0
18.5
__-
3 25.0
Negro
Negro
Negro
Negro
Negro
Negro
Japanese
Japanese
Japanese
Japanese
Japanese
Japanese
ICzeckanoweki, '06 _. - .. . 138 93 45
1604 1088 516
Totals and averages :
9
No. I No. %
American White
American White
American White
American White
American White
49
~
- .- d-
.
9
5
29.4
6.1 4.0
1
2.9
1
6.2 ?
9
42
2
5.0
3.9
1
2.9
- _ - - _ -
IWsgenseil, '27
Totals and averages:
Totals and averages :
.-
_-
.
Totals and averages :
458
I
17
14
3
I
40
35
5
; 373 283- 90
i 2118 1494 624
..___-.__
- -. -
- - - - 1 5 . 9 - .__
-.
. .
...
1 5 . 9 - -
- .-..
- ..
11
...
3.8
..
-.
.
4.0
2
__.-.11___.
. -
2.2
369 17.4 250 16.7 117 18.7
TABLE a
TOTAL NUXBIO or I)IDILI
I N W E l O E m S MUWAB WANTINO
T E E MUSCLE WAS AUlVOSTHKE W U I ” I N 0 A S TOLWWS:
9
d
No. %
No. %
47 18.8
25
19
21
30
95
20 20.6
22 15.4
36 18.8
125 18.3
67 18.3
17
6
15.2 22 25.9
23.5 1 6.2
16.1 1 7.7
18.0 6 26.1
17.3 30-22.0
20.0
16 12.5
5
I
1
1
1
2 33.3
1 25.0
2 15.4
7 9.7
2 5.9
14 10.9
3 2.3
3
3
4.7
3.1
1 5.0
1 9.1
1 20.0
-
-
1 9.1
7 10.8
2 6.6
11 9.8
2
2
2
1
3.1
4.0
2.9
6.2
-
-
9
2.7
7
3.4
.8
2.6 18 3.3
7 2.8
19.7 49 29.0
22.2
2 6.2
21.5
2 7.6
18.1 12 26.1
!90 21.3 225 20.7
65 23.8
5.3
9 2.5
7 3.2
2
1.3
162 22.2
92 21.3
70 23.5
6 2.4
2 1.9
4
3.0
43
8.6
15 7.0
28 10.3
10.2
4
2.2
3
2.3
1
2.0
2 1.1 2 1.6
-
-
48 13.5
37 14.4
11 11.3
9
6.5
3
6.6
6
53
3.3 36
1
-
2 16.6
2.4 17
1 9.1
5.0
3.8
-
2.9
13.61 9 6.5
2.2
254 12.0 173 11.7 81 13.0
4 8.3
5 11.1 36 13.0 14 7.5 22 24.4
579 18.5 383 17.6 196 19.0
3.3148 3.3 32 3.0 16 3.3
1 100.0
-
-- -- ~
------______
- -- -- 2 2.8 2 3.1 -- - 1 2.9 13.3 - 4 3.1 2 1.8 2 11.8 3 2.3 3 2.7 - 2 2.3 2 1 0 . 0 - ----- ------------ - - - - - ----- - - - - - - ----- - - -_______
-------_2 15.4
1 9.1
1 50.0
- 2 2.8 1 1.5 1
- -- -- -
1
1
2
65
43
56
61
8
14.3
-_._____
4 17.0
4
2
6
18
5
--_--
-
1 5.9
1 7.1-
12.5
12.9-
- ----- .-----
-
2 0.5 2 0.751 2.9 41 2.8 20
--_--- ----- 3.2151 2.4 33 2.3 16 2.5
459
AMERICAN JOVBNAL OF PHYBICAL ANTHROPOIINIY, VOL. XXV, NO. 8
33.3
25.0
23.1
12.5
7.3
35 13.5
3 13.6
1 50.0
2 10.0
2 50.0
2 33.3
3 75.0
1 7.1
-3 13.6
17 13.1
5 8.3
- -
27 12.0
8 23.8
3.5
6 4.6
4.1
4 3.8
6 3.1
4 2.9
2 5.0
2 6.2
4 3 . 3 1
T
6
6
24
-----_
2.4
22.8
23.2
20.3
19.1
19 5.2 11 5.1
-- -- - -
9
L14
45
58
73
9
No. 96
1.5
2
2.0
4.1
5.6
0.5
d
No. %
2
18.0
-
- 15 6.0 10 8.0 5 5.8
- 1 1.0 1 1.2 - - 6 4.2 6 4.6 - ----- --- 29 3.2 17 3.1 5 4.8
4.0 5.0 6.1 0.6-
5
4
8
1
d9
Yo.
1 0.9
3
- - _ - - _
d
9
\No.96 No. % No. %
1.2
1 100.0
1 33.3
1 50.0
-
5
4
8
1
Left aide only
Id0
3
230 14.3 154 14.1 76-15.7
1 18.3
dQ
d
9
lo. % No. % No. %
8.1
5.6 11
21 11.9
1
96
No.
37 17.1 30
7.0
1
Eight aide o*
Both aid-
d0
3.6
16 4.0
-2
2
T
4
5.0
3.8
1
2.9
_ _ _ _ - _
1 2.9
1 3.6
1 1.2
1 1.4
-
-
17 3.0
4
2.2
25
3.3
543 15.2 430 14.4 209 17.0
460
M. F. ASHLEY-MONTAQU
cause the article was written in Japanese and was accompanied
only by a very inadequate summary in German. It is fortunate
that with these few exceptions it proved possible to consult all
the other works referred to in this paper, and as one result of
this the writer has been, able to correct a number of serious
errors relating to the distribution of the pyramidalis in man
which occur in the literature.
Table 1presents the maximum number of facts which it has
been possible to derive from the available data relating to the
presence or absence of the pyramidalis on one or both sides
of the body in both sexes within the various groups therein
ad
-%
-
Rrcort
Fig. 1 Absence of the pyramidalis in Homo sapiens.
represented. Where it has been possible to compute totals and
averages this has been done. The most significant portion of
this table is for the most part restricted to the section presenting the findings for the total number of sides in which
the muscle was wanting. Here the findings are expressed as
a percentage of the total number of sides examined, i.e., treating each individual as possessing two sides. From the remainder of the table it is possible in some nineteen casep
representing various local racial groups, to gain some idea
of the sexual variation of the pyramidalis on both sides of
the body. Actually, these facts are more clearly and more
reliably brought out in table 2 in which only those groups are
SIGNIFICANCE O F PYRAMIDALIS MUSCLE
461
dealt with for which, with one or two exceptions, the complete
data are available. I n figure 1 a graphical analysis is presented of the findings set out in table 2 for the 2118 individuals (with the exception of twelve Mexicans whose racial
affiliation was not clear) of all the groups therein listed. This
figure simply illustrates the kind and degree of quantitative
and qualitative variation which occurs in the pyramidalis
muscle in the human species. It will be seen that in the species
Homo sapiens as a whole the pyramidalis is absent in 17.4%
of cases, or put the other way round, the pyramidalis is present on one or both sides of the body in 82.6% of cases. Thus
the pyramidalis is in man more frequently present than in
any other primate. Whether one o r both sides of the body be
considered it will be observed that the pyramidalis uniformly
tends to be more frequently wanting in the female than in the
male. It is possible that this sexual difference represents but
another reflection of the greater general variability of the
female as compared with the male. This interpretation, of
course, implies that the normal condition in the human species
is presence of the muscle, and that the female tends to vary
more frequently from the norm than the male. I t is, of course,
clear that local groups or races may deviate from such a rule,
and that males may exhibit a greater variability than the females, and while this may be true for many other characteristics as Karl Pearson (1897) showed many years ago, when
the species as a whole is considered it does appear that the
female tends to exhibit a greater variability than the male.
In the human species as a whole the pyramidalis is more frequently wanting in 2.6% of sides in females than in males, the
distribution in favor of the female being as follows, both sides
1.3%,right side 0.40/0, left side 0.270.
It will be noted that the pyramidalis is more than five times
more frequently wanting 011 both sides of the body than it is
011 one side of the body, either the right or left, and that the
muscle appeal's to be slightly more often waiitixig on the right
(2.9%,)than oil the left side (2.4%).
46a
M. F. ASHLEY-MONTAQU
These findings tend to hold good in a general way for most
of the races of mankind; there are, however, certain interesting exceptions to these generalizations, and certainly significant quantitative differences in the frequency of variation of
this muscle in different races. As a first step toward the description and understanding of these differences the &dings
presented in table 2 have been summated and averaged for
each of the three races into which the individual groups fall,
namely, White, Negro, and Mongol. Any more refined statistical analysis than that presented was quite impracticable
owing to the form in which most investigators have chosen to
present their data. The details of the composition of each of
these groups will be discussed later. The frequency percentTABLE 8
The frequency with which the pyramidalis was wanting among Whites, Negroes
and Mongols
.
_--___.-
TOTALNUYBEP
OF sUBJECT8WHICHTHE
I
I-
._
-.
.
T H E MUSCLE WAS ALTOQBTHEB WANTINQ:
Right aide
1
Left side
I
Total number
only
of sidea
--_
- only
d
9
d
P
d
P
d
/
P
d
P
d ?
.
.__-. --- _-
3.3
3.1
0.8
2.4 3.3 3.3 3.0,3.3 18.5 17.6 19.0
13.5 12.0 23.8
1.8 11.8 2.3 2.7;0.7 - 3.3 3.0 2.2
~
- I -
ages with which the pyramidalis was wanting in each of these
three groups is shown in table 3 and in figure 2. From these
it will be seen that the pyramidalis is more frequently wanting
in Whites (20.7%) than in Negroes (16.3%) and Mongols
(3.8%), but that the difference between Whites and Negroes
is only 4.476, whereas in the case of the Mongols the difference
is very considerably greater, being slightly more than five
times less frequently wanting than among Whites, and more
than four times less frequently than among Negroes. These
differences arc extraordinarily interesting. Whites and
Negroes further differ from Mongols in the fact that while in
Whites and Negroes the pyramidalis is as a rule more frequently wanting on one or both sides of the body in the female,
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
463
in the Mongols it would on the contrary appear to be more
frequently wanting in the males. But as a close perusal of
tables 1 and 2 will show such generalizations concerning the
sexual variation or absence of the pyramidalis in Whites,
Negroes, and Mongols do not always hold good for particular
local groups. Thus, it will be seen from tables 1 and 2 that
in Whites the pyramidalis may be more frequently wanting
among males than among females, depending upon the local
group considered. This is the case in two out of the five
Fig.2 Absence of the pyramidalis in Mongols, Negroes, and Whites.
American groups listed, in three out of the four French
groups, and in at least two out of the five Negro groups. It
will be also noted that among the Poles reported by Loth the
males appear to show a greater frequency of absence of the
muscle for all sides (28%) than the females (9.4%). Interestingly enough a similar variation is encountered among the
Mongols, for at least two Japanese groups (Okayama and
Hokuriku), and one Chinese group (reported by Wagenseil),
show this muscle more frequently wanting in the males than
in the females. Variation of this sort suggests that such dif-
464
M. F. ASHLEY-MONTAQU
ferences in the results obtained by various investigators in
different localities on a single ethnic stock are to be explained
principally as the reflection of genetic differences existing in
the various groups investigated. I n a study of the palmark
longus muscle in cadavers and families of living individuals
Thompson et al. ('21) have shown that the absence of the
palmaris longus is unquestionably a genetically determined
characteristic, and that its suppression or reduction in man is
probably due to a single dominant gene. These investigators
have little doubt that the discrepancies in the results of various workers respecting the frequency with which this muscle
is wanting in the different groups iuvestigated by them are
due to the genetic differences characterizing the groups concerned. It has already been suggested that this is the most
likely explanation of the differences encountered in the frequency with which the pyramidalis is found wanting in the
groups of mankind thus far investigated, and surely it is the
most obvious explanation.'
The evidence, as far as it goes, indicates that the pyramidalis is occasionally more frequently wanting among males in
certain local groups than among females, and that here again
we may possibly be dealing with the reflection of a genetic
factor, though in this connection it perhaps requires to be
pointed out that the various series at our disposal are generally not quite large enough for certainty. It need hardly be
pointed out, for example, that the height of the column for
Negro females in figure 2 is most probably greatly exaggerated owing to the small number of records (a total of seventeen) for Negro females. On the whole, however, there can
'It has, unfortunately, not proven possible to devise a method whereby living
individuals possessing a pyramidalis could be distinguished from those in whom
it is wanting. It is greatly to be doubted whether under the most favorable circumstances this could ever be done, for the peculiar anatomical relationships of
this muscle and i t 8 relatively small size are such a8 to render it, in the writer's
opinion, beyond the reach of ordinary methods of inspection or palpation. The
pyramidalis is therefore a muscle evidently unsuited for genetic study in the living,
a fact which is the more to be regretted since it is a muscle of much interest from
the evolutionary standpoint.
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
465
be little question that the muscle is generally more frequently
wanting in the female than in the male. It is of interest here
to note that in an exhaustive study of the incidence of the
psoas minor muscle in man Seib ( '34) found that this muscle
was significantly more frequently absent in White and Negro
females than in the males of each of these groups. Of interest,
too, are Seib's findings that the psoas minor was most often
absent in American Negroes (in 66.6% of 674 sides), least
often absent in Mongols (in 49.9% of 722 sides), and that
American and other Whites occupied an intermediate position
(absent in 57.1% of 4507 sides). Seib concludes on the basis
of the comparative anatomical evidence and the findings in
man that the psoas minor is in process of retrogression in the
primates generally and in man most strikingly. Such a conclusion would be quite consonant with the findings presented
in this paper for the pyramidalis which, it is clear, must be
definitely regarded in the opposite light, not as a retrogressive but as a progressive structure in the lower primates and
most strikingly in man.
In addition to the groups listed in the tables the pyramidalis
has been found wanting in two Papuans dissected by SteffensKorner ( 'lo), and present in one Papuan dissected by Forster
('04) ; the pyramidalis was wanting in a Carib dissected by
Chudzinski (1898),and in two Fuegians dissected by Hyades
and Deniker (1891). It was present on the left side only in
a Peruvian dissected by Chudzinski, and in an Australian
aboriginal dissected by Loth ('31). The muscle was present
in a Korean dissected by Ch0uk6,~and in a Chinese adult male
dissected by the present writer at New Pork.
Among the many interesting facts brought out in table 2
one of the most striking is the remarkably close agreement
of the findings for the American Whites with those for the
English group. The agreement is so close for each of the
varying conditions considered that the conclusion seems to
be inescapable that the American White group of table 2 was
made up predominantly of individuals of English extraction,.
' Personal communication ( '39).
466
M. F. ASHLEY-MONTAGU
or perhaps it would be more correct to say that genetically
both groups, the American White and the English, were very
closely related. Particularly interesting is the fact that the
group of American Whites most closely resembling the English in respect of the characters of the pyramidalis is the
group from New England Cambridge, a region in which the
population was, in 1895 at least, still for the most part of
English origin. The New York, Denver, and Chicago American
Whites exhibit slight deviations from the Cambridge and English groups such as we would expect to find in any more or less
fairly homogeneous series. The Denver and Chicago groups
show a remarkable reversal of the &dings independently obtained by Dwight (1893) and the present writer in connection
with the sexual distribution of the variations of the pyramidalis. Whereas Dwight (1893) and the present writer found
the pyramidalis in American Whites uniformly more frequentIy wanting in the female than in the male, Choukg ('35)
and Anson et al. ( '38), and Beaton and Anson ('39) found it
uniformly more frequently wanting in the male. I n the English series all the investigators reporting found that the pyramidalis was invariably more frequently wanting in the
female than in the male. In all human groups of whatever
race, as we have already seen, such differences in sexual frequencies have been found, a fact which is perhaps better
brought out in table 1 than in table 2. Local genetic differences must for the present, at any rate, be called in as the
'deus ex machina' to account for such sexual variation in the
distribution of the pyramidalis muscle within each racial
group.
Upon the basis of our observations thus far we may present
the following conclusions :
1. The pyramidalis is a muscle which occurs in all varieties
of man as a normal feature of their anatomy.
2. I n the species Homo sapiens as a whole the pyramidalis
is present in 82.6% of cases.
3. The frequency differences in the presence or absence of
the pyramidalis in any one group, racial or local, are primarily
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
467
the reflection of genetic differences, the muscle probably being
inherited in each group as a simple dominant.
4. The pyramidalis tends to be slightly more frequently
absent in the female than in the male.
5. The evidence of comparative anatomy strongly suggests
that the pyramidalis of man is a structure inherited from the
anthropoid ancestor common to the Great Apes and to man.
6. The kind of variability exhibited by the pyramidalis in
man does not lend the slightest support to the suggestion that
it is a retrogressive structure in process of disappearing in
the human species, but on the other hand the evidence very
strongly indicates that this muscle is a progressive structure
now more firmly entrenched as a normal part of man’s anatomy than it can have been at any other time in the evolution
of the primates.
THE VARIATIONS IN FORM AND STRUCTURE OF THE
PYRAMIDALIS IN MAN
Thus far we have discussed the variability of the pyramidalis in terms of presence or absence of the muscle on one or
both sides of the body, and only very briefly referred to the
variations which it exhibits in form and structure; we may
now consider the latter aspect of pyramidalis variation in
somewhat greater detail.
Various investigators, we have already had occasion to
observe, have claimed to have found two pyramidales on one
or both sides of the body, namely, Riolan (1626), Ruysch
(1704), Verheyen (1706),. Santorini (1724), Winslow (1749),
Hammer (1765), Lietaud (1782), and Hallett (1848);or even
three and four (Hornex-, 1826), while other anatomists who
have looked for such anomalies have generally failed to find
them. I n the 226 bodies dissected by the writer no instance
occurred of a completely doubled, tripled, or quadrupled
muscle, although in one case, a white male, the pyramidalis on
each side arose from the symphysis pubis by two distinct heads
which united immediately above the level of the pubic crest
to form the single belly of the muscle on each side. I n this
468
M. F. ASHLEY-MONTAQU
particular case the muscle was extremely well developed measuring some 110 mm. in length and 31 mm. in breadth a t its
origin. Anson et al. (’38) out of a total of 163 bodies dissected at Chicago found one ease of doubling, the muscle being
bilaminar, the strata fused a t the extremities. Since in most
cases we do not have any statement of the frequency within
a series with which duplicated pyramidales were encountered
by those who have reported such cases, it, is impossible to say
anything very definite concerning the frequency with which
such cases occur in man, but it does appear fairly evident
that this form of variation is relatively rare.
I n an opposite direction a more common condition is encountered in the extreme attenuation of the muscular fibres
composing the pyramidalis. In such cases the muscle is represented by tendinous fibres, either admixed with f a t and/or
fascia with or without the presence of some muscle fibres
(Chudzinski, 1898; Vallois, ’26). Out of 165 bodies Anson et
al. (’38) found only one case in which the pyramidalis was
represented by “ a very thin tendinous band containing only
a few indistinct muscle fibres.” I n my own series of 226 bodies
in which the pyramidalis was present in 186 cases, 8 of these
cases, or in 4.3%, the pyramidalis was tendinous or almost
entirely so. The distribution and the details relating to these
eight cases were as follows:
AYEBICAN WHITES
Very fibrous, with a few muscle fibres present
Very fibrous, no muscular fibres demonstrable
Very fibrous, strongly adherent to transversus abdominis
Very fibrous, strongly adherent to transversus abdominis
Fibrous, with a few muscular fibres
Fibrous, with a few muscular fibrea
Very fibrous, no muscular fibres demonstrable
LENOTH
70 mm.
35
67
62
BBEADTH
14 mm.
8
22
22
68
17
65
60
20
5
70
6
NEQRO
d Fibrous, no muscular Abres demonstrable
Where, as in such case, the pyramidalis is for the most part
tendinous or entirely so it is generally thin, reduced in breadth,
but does not appear to suffer any considerable diminution in
length.
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
469
With respect to the dimensions of the pyramidalis the present investigator's hdings are presented in tables 4, 5, and 6.
Length of the muscle was measured with a sliding compass
from the origin of the tendinous fibres of the pyramidalis on
the pubis at the symphysis, and breadth was determined,
usually at the same place, at the maximum transverse diameter of the muscle. I n order to reach the origin of the pyramidalis at the pubis it is always necessary to dissect away all
the overlying tissues, and this, there is some reason to believe,
has not been the practise followed by most previous investigators who have probably followed the procedure of Loth
('12) who measured the length of the muscle from its muscular commencement and not from its tendinous origin. I n the
case of Anson et al. ( '38) it is expressly stated that the measurement of height (length) was taken from the pubic crest.
This difference in procedure and the fact that most of these
investigators have lumped their measurements for different
races and the sexes together probably accounts for the fact
that my own figures for total length of pyramidalis generally
exceed by about 11.0 mm. those of other investigators listed
in table 7. From tables 4 and 5 it will be seen that the average
length and breadth of the pyramidalis is, in both Whites and
Negroes, greater in males than in females, and further, that
the pyramidalis is larger in Whites in both these dimensions
than in Negroes, the smallest muscles occurring in Negroes
and the largest in Whites. These differences in the dimensioiis
of the pyramidalis between Whites and Negroes are probably
somewhat exaggerated, since only 31 Negro pyramidales were
examined as compared with 154 White pyramidales.
My own finding that the pyramidalis is generally larger in
the male than in the female is not in agreement with the finding of Vallois ( '26) on a series of fifty-six (27 8 , 29 9 ) French
bodies from Toulouse. Vallois found both the length and the
breadth of the pyramidalis greater in the female than in the
male. With the exception of breadth it is not possible to determine the differences in length between the sexes from
Vallois' figures ; the average breadth of the pyramidalis in
470
M. F. ASHLEY-MONTAQU
TABLE 4
Arithmetia mean and standard deviation of the diameters and indices
of the pyramidulis
~
RACE
i
N U Y B S B OT
OBSREVA81s T I O N E A N D
INDIVIDUAU
EXAXINID
8
P
~d
Negro
P
RACE
White
Negro
Difference
11
!
83.5
72.3
76.3
59.2
137
17
27
4
mm.
29.7 2 10.0
25.8 2 6.2 I
f 15.0 19.2 2 4.9 26.4 f 8.1 I
2 23.5 17.7 2 6.5 32.3 & 11.2
f 17.7 21.8 f 6.0
2 12.4 18.5 2 4.2
lG.7 f 6.1
13.9 f 5.7
15.2 2 5.0
11.5 f 7.3
~
TABLE 6
LENOTH
d
m&u
I
Minimum I Maximum
~____
83.5 2 17.7
76.3 2 15.0
7.2
I
1 1
158.0
100.0
58.0
35.0
30.0
5.0
d
RACE
White
Negro
Difference
INDEX
100 1 BRKADTH X WLNQTR
100
LENOTE
~
mm.
White
i
INDEX
BREADTR X
DRUDTH
LENOTH
_-___
?
_
_
_
.
I
-
m&u
1
I
I
72.3 2 12.4
59.2 -C 23.5
13.1
21.8 -C 6.0
19.2 f 4.9
3.6
8.0
6.0
2.0
I-~
Minimum I Maximum
50.0
27.0
23.0
'
100.0
85.0
15.0
BRKADTH
1 E:
I
0
-
-~
Minimum
m & r
1 I
..-
Maximum
32.0
.
m&u
18.5 2 4.2
17.7 2 6.5
0.8
I 1 265:;
'
Minimum Maximum
____--
;::1
;
-5.0
-1.0
z
it zx
al
Length
lWhite1
Negro
Rrendth
- - X 100
Length
Breadth X length,
-,White'
loo
\Negro!
Index
1
d
6
E
x
83.5
76.3
.
d 1 16.7
8 ! 15.2
I
-
al
$
K
a
__
35.0
30.0
8.0
6.0
158.0
100.0
55.0
27.0
White! 9
Negro' 9
White' Q
Negro: 0
72.4
59.2
18.5
17.7
50.0
27.0
3.0
10.0
100.0
85.0
2.5.0
26.0
13.7
8.5
100.0 White: Q
50.0 'Xegro p
25.8
32.3
8.0
16.6
34.0
48.2
13.9
3.0
11.5
3.5
24.0
22.1
.- -
2.8
4.8
_.___
. .
__
..
-
..-
INVSSTIOATOB
Nakamura
Japanese Kyusliu
Averanea for a11 erouus
Ashley-Montagu
American
Negro New York
120
-1320
31
126
67.7
66.7
67.7
66.9
103.0
116.1
100.0
115.0
19.2
-
57.0
27.4
18.9
18.4
18.2
20.3
16.0
20.2
18.6
20.3
mm.
-
Lverage
27.0
5.0
30.0
lOO.O+
71.7
118
Valloie
Ansoii et al.
Toulouse
French
20.0
138.0
62.0
5.0
400
120.0
mm.
40.0
Loth
American
Negro Cliieago
Warsaw
Polish
mm.
95.0
Minimum
35.0
63.9
67.1
mm.
Average Maximum
--
158.0
228
110
a+0
LlNaTH
77.9
Ashley-Montagu
American
White New York
TOTAL
NUMBER
or SrnlS
187
Amou et al.
Cambridge Barclay Smith
~
LOCALITY
American
White Chicago
English
OROUP
RACB OB
6.0
10.0
6.8
27.0
30.0
39.5
28.4
27.5
27.2
27.2
28.3
-
5.0
25.8
10.0
30.0
35.0
26.0
29.1
30.4
5.0
5.0
-
mm.
LBNOTH
YOBPWOUMIOAL INDEX
B a s d D T H~.
X 100
55.0
60.0
-
mm.
Maximum Minimum
BRLADTR
TABLE 7
The length and breadth of the pyramidalis in man
472
M. F. ASHLEY-MONTAOU
Vallois’ series is 20.3 mm. for males and the same amount for
females. Vallois considers that this superiority in size of the
female pyramidalis in his series over that of the males is
associated with the lesser cranio-caudal (xiphi-pubic) extent
of the female abdominal parieties, a suggestion the cogency
of which it is difficult to follow, particularly since Vallois
has himself failed to find any correlation between the diameters of the abdomen and those of the pyramidalis. Beaton
and Anson (’39) in the 354 muscles examined by them found
that the pyramidalis is larger in the male (length 65.2 mm.,
breadth 18.6 mm.) than in the female (length 62.5 mm., breadth
16.8 mm.).
It is to be regretted that most investigators have failed to
record the measurements of the pyramidalis in the sexes in
TABLE
LINQTH
RACE8 IN
QENEUL
63
92
53
(62)
(92)
(39)
YELLOW
BLACK
_
mm.
Avernge
Maximum
Minimum
8
_
_
mm.
mm.
77
92
53
(56)
(84)
(39)
WHITE
~
73
92
61
(72)
(92)
(54)
-. -. -.-.-
mm.
61
73
53
(59)
(71)
(51)
Chuctzinski’s measurements of the length of the pyramidalis from its tendinous
origin (unbracketed figures) and from its muscular commencement (bracketed
figures) on a total of twenty-ax subjects.
the different groups examined by them; we are fortunate however in having some records of average diameters and ranges
of these for several groups and races, and these have been
drawn up in table 7. Although my own measurements figure
in table 7 they have not been utilized in arriving at the averages shown in the last row of figures for the reason that the
measurements made by myself were made from the tendinous
origin of the muscle upon the anterior aspect of the pubis,
whereas those of the other investigators were made from the
muscular commencement of the pyramidalis a t the level of
the pubic crest. Chudzinski (1898) is the only investigator of
the pyramidalis to give the measurements of this muscle both
from its tendinous origin and from the commencement of its
muscular belly. His figures are given in table 8, the unbrack-
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
473
eted figures representing the measurements from the tendinous origin of the muscle and those enclosed in brackets the
measurements of the muscle from its muscular commencement. Chudzinski’s figures are based upon the examination
of twenty-six individuals whom he classified as Black, Yellow,
and White. The Black group consisted of 108 and 5 P
Negroes from the French possessions in Africa; the individuals comprising the Yellow group consisted of a Peruvian,
an Annamite, a native of Pondicherry and a Senegalese.
The White group was made up of six Frenchmen and one
Arab. The group which he refers to as ‘Races in General’ is
made up of these twenty-six individuals.
From table 8 it will be seen that except in the case of the
Black group the disparities between total muscle length and
fleshy muscle length are not very marked, a fact which is no
doubt due to the small number of cases examined by Chudzinski. I n the series of 85 muscles measured by Anson et al.
( ’38) the inferior extension of the pyramidalis upon the ramus
of the pubis was 5.0 mm. to 9.0 mm. in 17 cases, 10.0 mm. to
15.0 mm. in 50 cases, and 16.0 mm. to 20.0 mm. in 18 cases.
These observations together with my own suggest that the
pubic portion of the pyramidalis has a mean extent of some
12.0 mm. in the races of man with a range of between 5.0 to
20.0 mm. Chudzinski’s (1898) figures for breadth of pyramidalis in his series are as follows :
BLaOX
Average
Maximum
Minimum
22 mm.
32
12
YLLMW
34 mm.
37
30
From table 7, from which Chudzinski’s figures have been
omitted because his series is both too small and too mixed,
it is evident that the average length of the pyramidalis in the
species Homo sapiens is 66.7 mm., with a range of 27.4 to
116.1 mm., and that the average breadth is 18.9 mm. with a
range of from 6.8 to 39.5 mm.
My own measuremeiits on the pyramidalis were, as a rule,
made on the right side of the body, or where the muscle was
474
M. F. ASHLEY-MONTAQU
absent on that side, on the left pyramidalis. Several other
investigators have measured this muscle on both sides of the
body, and from their observations it is clear that asymmetries
occur. We have already seen that as early as 1626 Riolan
had noted that the left pyramidalis is sometimes smaller
than the right. Hallett (1848) writes of his own series of 200
Scottish bodies that one pyramidalis “was found to be always
larger than the other, and this was most frequently the case
with the right.” This is almost certainly an exaggeration.
Barclay Smith (1895) writing of observations made on a
series of English bodies (50 8 and 5 P ) examined at Cambridge states that “ I n only fifteen cases, (30 per cent), where
two muscles were present, were the muscle slips of the two
sides symmetrically disposed, while in twenty cases (40 per
cent) they were asymmetrical. The muscle was much more
frequently absent, and tended to be much less symmetrically
disposed in the female than in the male . . . on the other
hand, when the muscle was found on both sides, but asymmetrically disposed, it was more frequently (about 2 to 1)
found to be larger on the right side than on the left side.”
Warden (1895) states, in connection with a series of Scottish
bodies (21 8 , 9 9 ) examined at Glasgow, that extreme asymmetry of the pyramidalis was observed in several cases, and
that in two female subjects the muscle measured 5 t and 4 inches
in length, “the left portion of the muscle being the larger
by 2$ inches in one case and 1 inch in the other.” Anson et al.
(’38) found that in the fifty bodies examined by them (100
muscles) “twenty bodies show muscles of the same or approximately the same height (i.e., within 2 mm. of exact correspondence) ; in fifteen bodies the muscle of the right side is longer,
in fifteen that of the left.”
From these several observations we may conclude that the
pyramidalis may vary in size on opposite sides of the body
in the same individual, and that in such cases the larger muscle
may be present on the left side as often as on the right. With
respect to variations in form and attachments Verheyen
(1706) has described a case in which the pyramidalis was
.
475
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
Fig.3 The variety of forms of the pyramidalie in the author'a series of 137
white males.
1d
B
3
Fig.4 The four extreme types of pyramidalis in the author's aeries of 137
white malee, compared with an average proportioned. Pgramidalie ( V ) .
APBRIOAN J O U B B A L OB PHYSICAL ANTHMPOII)(IY, V m . XXV, NO. 9
476
M. F. ASHLEY-MONTAOU
RACE
I
1
rn
SHOBT
Y6DIUY
-_
-~
White
Negro
1
31.4
d
P
s
9
47.4
41.4
[
--
18.5
75.0
1
1
i
j
47.1
;
25.0
I
59.3
LONO
21.2
11.6
22.2
-
right side crossed to the left to be inserted into the deep surface of the body of the left pyramidalis. The forms of the
pyramidalis and the frequencies with which they were encountered in the present writer’s series of 137 male white
bodies are set out in figures 3 and 4 and in table 9. The diagrams in these figures represent the lengths and breadths of
the types of muscles encountered, but do not present the
actual orientation of the muscle on the abdominal wall. The
vertical side in the diagrams represents, of course, the medial
border of the right pyramidalis ; in reality the medial border
of the pyramidalis is rarely so vertically oriented, although
it is generally appreciably more vertical than the lateral border. I n figure 3 the intermediate and lateral lines convey a
fairly accurate representation of the obliquity of the lateral
border of the pyramidalis within the body. The intermediate
SIGNIFICANCE O F PYRAMIDALIS M U S C L E
477
line in each diagram in figure 3 is intended to represent the
lateral border of a single right-sided muscle as also the medial
border of a single right muscle falling within the range of
the particular length-group figured but characterized by a
different breadth. It will be seen from figure 3 that when averaged 31.4% of muscles ranged in length (height) between 49.0
mm. and 63.0 mm. (short), 47.4% between 72.5 and 82.5 mm.
(medium), and 21.2% between 91.0 and 108.0 mm. (long). The
average length of the muscle in this series being 83.5 mm.,
the average muscle thus falling into the class between medium
and long, a fact which would hardly suggest a muscle in process of retrogression. The pyramidalis in man, it would appear
then, is generally a relatively large muscle, that is to say, it
is more often than not large than small. I n table 9 are given
the frequencies with which the various types of pyramidalis,
according to size, were encountered in the writer’s series of
154 American Whites and 34 American Negroes. I n figure 4
the four extreme types of muscle found in the writer’s series
of 137 American White males are presented. Owing to the
small number of Negroes represented in table 9 it is hardly
possible to draw any very definite inferences from the figures
presented in the table for that group; taken as they stand
these figures suggest that the pyramidalis in Whites tends to
be both longer and broader than in Negroes, large muscles
occurring more frequently among the former than among the
latter. Chudzinski (1898),on the other hand, in his series of
fifteen Negroes compared with seven Whites found that the
total length of the muscle was greater in Negroes than in
Whites, but that the fleshy extent of the muscle was less in
the Negroes than in Whites. Beaton and Anson (’39) find
that the pyramidalis is shorter (length 63.9 mm.) and wider
(18.6 mm.) in Whites than in Negroes (length 16.9 mm.,
breadth 18.2 mm.).
Measurements of the pyramidalis in infra-human primates
are extremely few. Sperino (1897)recorded the length of the
muscle in the adult chimpanzee examined by him as 110 mm.
478
,M.
F. ASHLEY-MONTAQU
R6zycki ( ’22) recorded the length and breadth of the pyramidalis in three juvenile chimpanzees as follows :
No. 2
No. 3
No. 4
Right side
Left side
Left side
Breadth 9.0 mm.
Breadth 8.0
Breadth 8.0
Length 70.0 mm.
Length 80.0
Length 108.0
Index 13
Index 10
Index 7
I n the last case (no. 4) the pyramidalis reached the level of
the umbilicus. I n the other two cases the level reached was
as f a r as the second tendinous intersection of the rectus, as
is normally the case in man. It is of interest to note here that
in the majority of the catarrhine monkeys investigated by
Tschachmachtschan (’12) that in no case was the pyramidalis
observed to reach to the level of the umbilicus ; this investigator writes “ I n den meisten Fallen erreicht die obere Spitze
des M. pyramidalis nur die unterste Inscriptio tendinea, kann
noch hoher bis zur der zweiten und dritten Inscription emporsteigen, erlangt aber nie den Nabel” (p. 351).
Measurements of the pyramidalis in the infra-human primates have been made but not published by Loth (’31). Loth,
however, does give indices based on the length and breadth
of the pyramidalis, breadth being multiplied by 100 and the
total sum thus obtained divided by length. This procedure
yields a morphological index of the pyramidalis expressing
the relation of the breadth of the muscle to its length, and thus
providing some idea of its actual form. The number of muscles upon which Loth’s indices are based is not stated, but his
findings are as follows:
Morphological index of the pyramidalis
Platyrrhinae
Catarrhinae
Hylobatee
Pan
37-42
20-30
13
12.7
The higher indices, 30-42, indicate muscles which are relatively broad in relation to their length, the lower indices
muscles which are relatively narrow in relation to their length.
Reference to table 7 will show that the average morphological
index of the pyramidalis in Homo sapiens is 27.5 with a range
of 25.8 to 30.4,from which it would seem that the form of the
SIGNIFICANCE O F PYRAMIDALIS MUSCLE
470
human pyramidalis most closely resembles that of the catarrhine monkeys, and that in the anthropoids the muscle appears to be relatively longer in relation to its breadth than
is the case either in the monkeys or in man. Further observation of the diameters of the pyramidalis in the anthropoids
may require some modification of this view, for it does appear
that at least in length (height) the anthropoid pyramidalis is
very like that of man. The variations in the morphological
index of the pyramidalis presented in table 7 do not appear
to be very significant. I n general it would seem that the
pyramidalis of Whites is somewhat broader in relation to
length than is the case in Negroes. Upon the evidence it
would seem reasonable to suppose that the pyramidalis of
man has either undergone a secondary elongation in breadth
and a slight reduction in length since the original separation
from the common stock which culminated in the two lines
leading to the anthropoids on the one hand and to man on the
other, or else that man has retained the catarrhine proportions of the muscles which may have characterized the ancestral group of the hominidae. The fact is, however, that man
displays morphological indices of the pyramidalis which range
from 3 to 81, thus exhibiting every variety of form of the
pyramidalis which is known to occur among the primates.
Here once more man shows himself a primate of the primates.
That the hominid pyramidalis could easily have been derived
from a catarrhine or anthropoid type of pyramidalis should
be clear.
The complete evidence strongly suggests that the human
pyramidalis is a muscle of lower primate origin, and that its
modifications have been restricted to a slight decrease in
length and a compensatory increase in breadth. In many
cases the form of the muscle and its proportions in no wag
differ from the same characters in the lower primates, so that
we may reasonably draw the conclusion that the variations
in form and proportions which the human pyramidalis displays merely follow the patterns of the muscle which are to be
found among the infra-human primates. This, principally, is
480
M. F. ASHLEY-MONTAGU
the significance and the meaning of the variability in form and
proportions of the human pyramidalis.
A final method of testing the truth of the above conclusion
would be by attempting to discover whether or not the form
of the pyramidalis is determined by or is to be regarded as
in any way related to the form or diameters of some other
structure or structures of the body. If it could be shown, for
example, that some such relation holds good, then our hypothesis concerning the meaning of this variability of the
form of the pyramidalis in man would have to be considerably
modified. Czeckanowski ( '06) in a general study of the relations of muscle variations could not find any significant relation between the variability exhibited by the pyramidalis and
that of any of the other muscles studied. Vallois ('26) could
find no relation between length of pyramidalis and stature,
and similarly Anson et al. ('38) failed to find any relationship between these two characters. Vallois, moreover, could
find no relationship between the proportions of the anterior
abdominal wall (measured from tip of xiphoid process to
pubic crest, and from the left anterior superior iliac spine to
the right anterior superior iliac spine). The present investigator without knowing of Vallois' work investigated the relationship of the pyramidalis to the form of the abdominal wall,
taking exactly similar points for his measurements of the
abdomen as Vallois had done, i.e., height of abdomen was
taken from the inferiormost border of the xiphoid process to
the superior border of the pubic crest, and breadth of abdomen from the left to the right anterior superior iliac spines.
These measurements were made with a metal flexible tape
graduated in millimeters. From these measurements abdominal indices were computed by multiplying the abdominal
breadth by 100 and dividing the resulting figure by abdominal
height. The abdominal index may be taken to be a fairly reliable expression of the form of the anterior abdominal wall.
The coefficient of correlation between the abdominal and morphological pyramidal indices in 226 cases was found to be
( r ) 0.10, thus showing that there is not the slightest relation-
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
481
ship between the form of the abdomen and the form of the
pyramidalis. The coefficient of correlation between the abdominal index and the length of the pyramidalis was (r)
-0.14, again showing no relationship whatsoever ; similar
non-relationship was found to be true for height of abdomen
and length of pyramidalis in which the coefficient of correlation was ( r ) 0.11.
It now remained to discover whether the length of the
pyramidalis was in any significant manner correlated with
its breadth, and here again it was found that there was no
relationship whatever between these two dimensions, the coefficient of correlation being ( r ) 0.0409. Hence our conclusion
remains unmodified that the human pyramidalis owes the
characters of its variability, or the variability of its characters, to the genetic patterns of the pyramidalis inherited from
its lower primate ancestors.
DISCUSSION
We have seen in the course of this paper that the pyramidalis is in man a more constantly developed muscle than is
the case in any other primate. This being so it would appear
that such constancy of the pyramidalis as is found in man
represents a relatively recent primate character, for which
there no doubt exists some reason. In this connection a fact
of functional interest which at once recommends itself to our
attention is that this constancy is significantly associated with
the erect posture of man. Vallois (’26)has already referred
to this association. Referring to the progressive differentiation of the pyramidalis in the primates culminating in man
Vallois writes :
La cause de cette diffhrenciation proFessive du pyramidaI
humain est difficile B prhciser: je considhe comme possible
qu’elle soit li6e B 1’Qlargissementprogressif du bassin et B la
modification de la sustentation dam la paroi abdominale ;
1’Homme est, en effet, de tous les Primates, celui dont la
largeur abdominale, au niveau du bassin, est la plus grande.
(Villemin). Peut-8tre cette disposition, jointe B 1’existence.
de la station verticale, necessite-t-elle uiie action plus con--
482
M. F. ASHLEY-MONTAOU
siderable de la part des muscles larges de l’abdoment Or, la
presence du pyramidal, en favorisant la tension de la ligne
blanche, f acilite Qvidemment la constriction de ces muscles
larges et augmente la solidite de la paroi. Ce n’est lir, naturellement qu’une hypothhe, mais elle me semble appuy6e par
ce fait que le pyramidal est plus frequent chez la femme et,
quand pr6sent, plus long chez celle-ci: or la femme, en raison
de la grossesse, a certainement besoin d’une paroi abdominale
plus resistante que l’homme.
Since, as we have already seen, there is good reason to believe that females in general more frequently lack a ppramidalis than males, and that the muscle is generally smaller in
females than in males, it would hardly seem likely that the
presence of this muscle in the human species in so high a degree of frequency has any connection whatsoever with the
functions of pregnancy in the female. As for Vallois’ suggestion that during pregnancy the female needs a more resistant abdominal wall than the male normally does, it might
with perhaps greater force be argued that the female has much
less need of a resistant abdominal wall than the male, for
surely it is clear that what she needs is an abdominal wall
which readily adapts itself to the progressive increase in size
of the uterus during pregnancy. Vallois’ other suggestion
that the constancy of the pyramidalis in man is associated
with the progressive enlargement of the pelvic basin and the
erect posture has a much greater appeal. Certainly the rearrangement which the pelvic musculature has undergone with
the adoption of the erect posture by man calls for a re-enforcement of the abdominal wall, and every possible structure
already present in this region of the body would mechanically
be called upon to assist in its strengthening. It is assumed
that having become a muscle functionally more necessary than
it is iii any of the other primates the pyramidalis has in man
respoiidecl by becoming a relatively constant part of his
anatomy. It is of interest to note here that not only is the
constancy of the pyramidalis associated with the erect posture, but it is even more significantly associated with the form
of the pelvis, remembering that the changes in the f o r m of
SIGNIFICANCE OF PYRAMIDALIS MUSCLE
483
the pelvis are for the most part actually due to the assumption of the erect posture, and that from the lowest primates to
the highest there has been a progressive increase in the breadth
of the pelvis and a reduction in its height (Martin, '28 ;Straus,
'29; Schulta, '30; Reynolds, '31). It would appear probable
that the establishment of the pyramidalis as a normal feature
of man's anatomy is due principally to factors similar to those
which have brought about the widening of the human pelvis,
factors demanding a re-adaptation of the pelvic and abdominal structures to the new stresses and strains put upon them
by the development of the orthograde posture in man. A
condition which may have some bearing upon this matter is
the fact that in man the posterior lamina of the sheath of the
rectus is deficient below the level midway between the umbilicus and the pubic crest, the rectus in this region lying directly
upon the fascia transversalis. According to Sommer ( '07)the
gorilla exhibits a similar deficiency, though Bisc'hoff (1879)
did not observe it in the gorilla which he dissected. I n the
orang according to Sonntag ('24) the deficiency is well
marked. The orang, it will be recalled, does not possess a
pyramidalis, hence it does not seem very likely that this deficiency of the posterior wall of the rectus sheath is intimately
associated with the progressive development of the pyramidalis, although in man, in whom the lower part of the abdominal wall is notoriously weaker than in any other primate the
association may be a more significant one.
SUMMARY AND CONCLUSION8
I n the present study an attempt has been made to solve
several problems relating to the presence and variability of
the pyramidalis muscle in man. It has been generally believed that this muscle is in man the vestige of a prototherian
structure of which it is the degenerate homologue, that is, of
the pyramidal muscle which is so well developed in the marsupials. It has been shown that the human pyramidalis is at
most an analogous structure to the marsupial pyramidal, and
that since the marsupials are not in any sense to be regarded
484
M. F. ASHLEY-MONTAOU
as standing ancestrally in the direct line of evolution
of the mammals they cannot possibly have contributed this
muscle to the general structure of the Hominidae. Moreover,
practically all non-primate mammals, with the exception of
the insectivores, lack a pyramidalis, and since this muscle does
occur among some of the menotyphlous insectivores, a group
which may be regarded as ancestral to the primates, it is
reasonable to assume that the primate pyramidalis owes its
origin to its common possession by an insectivore ancestor.
The Lemuroidea do not possess a pyramidalis, but many of
the Tarsiers, Platyrrhinae, and Catarrhinae do, as do the
Anthropomorpha with the possible exception of the orang.
Since both of the African apes possess this muscle it is again
reasonable to assume that the stock from which they together
with the Hominidae arose possessed a pyramidalis, and hence
it becomes quite unnecessary to postulate a more remote or
primitive ancestry, as some have seen fit to do, for the human
pyramidalis than such an ancestral anthropoid stock.
It has been shown that the diameters of the pyramidalis
correlate neither with stature, nor with the form of the abdomen or any of its diameters, nor as between the diameters of
the muscle itself. It has further been shown that the variability in the form of the pyramidalis in man is merely a reflection of the variations in the form of this muscle which are
encountered among the lower primates.
The differences in the frequencies with which the pyramidalis is absent in various groups and races of mankind have
been examined, and it has been shown that these are probably
due to local genetic differences in the groups and races examined.
Among the races of mankind as a whole the pyramidalis is
absent in 17.4% of cases. It is present on one or both sides of
the body in 82.6% of cases.
The pyramidalis appears to be slightly more frequently
wanting and more generally variable in females than in males,
and tends to be rather larger in Whites than in Negroes.
SIGNIFICANCE O F PYRAMIDALIS MUSCLE
485
Finally, the evidence indicates quite clearly that far from
being a muscle which is tending to disappear in man, the
pyramidalis is on the contrary a muscle which must be regarded as a progressive character which has long been most
firmly entrenched as a constant feature of the normal structure of man.
ACKNOWLEDQMENTS
I am greatly indebted to Dr. K. S. Chouke of the Department of Anatomy of the University of Colorado, Denver, f o r
his kindness in sending me the results of his findings on the
pyramidalis and which he has allowed me to use i n the present
paper; and for a similar kindness, before the publication of
their own papers on the pyramidalis, I am greatly indebted
to Dr. Lindsay E. Beaton and his colleagues Drs. B. J. Anson
and C. B. McVay of the Department of Anatomy of Northwestern University Medical School, Chicago. To Dr. Klara B.
Scheib of New York I owe many thanks for her able assistance
in the laborious task of statistically analyzing my data, though
for the final form of that analysis and the computation of all
other constants, figures, tabular matter and the graphical
analysis, I alone am responsible.
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ADACHI,B. 1910 Beitrage zur Anatomie der Japaner. XII. Die Statistik der
Muskelvarietaten. Z. Morph. & Anthrop., XII, 261-312.
AITXEN, A. B. 1912 Note on the insertion of the rectus abdominis muscle.
G l ~ g o wMed. J., LXXVIII, 171-172.
ALIX, P. H. E. 1804 Snr les muecles pyramidaux. Bull. Soc. Philomatliique
Parb, 129.
1865 Nouvelles observations mr la mgologie du Tarsier. Bull. SOC.
Philomathique Parie, 168-177.
1867 Sur l’appareil locomoteur de 1’Omithorynque et de 1’Echidiie.
Bull. Soc. Philomathique Paris, 170-218.
1867 8ur l’appareil locomoteur de la R o w e t t e d’Edwards (Pteropue
Edwardeii) Bull. Soc. Philomathique Paris, 127-159.
ANCEL, P. 1901/02 Documents receuillies B la salle de dissection de la Fac. de
MBdeeine de Nancy. Bibliograpliie Anatomiqne, I X & X, 133-160,
124-138.
~ N S O N , B. J., L. E. BEATON
AND C. B. MCVAY 1938 The pyramidalis muscle.
Anat. Rec., LXXII, 405-411.
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