Anthropological significance of the musculus pyramidalis and its variability in man.код для вставкиСкачать
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 .................. ..................................................... ....................................................... .................................... ............................................. 435 436 437 442 448 448 448 449 450 451 451 451 452 452 452 453 457 467 481 483 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. 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