Nomenclatural review of long digital forelimb flexors in carnivores.

THE ANATOMICAL RECORD 216:471-473 (1986)
Nomenclatural Review of Long Digital Forelimb
Flexors in Carnivores
C.F. SPOOR AND D.M. BADOUX
Department of Paleontology and Stratigraphy, Znstitute of Earth Sciences, State University of
Utrecht, Budapestlaan 4, I? 0.Box 80.021, 3508 TA {C.F.S.), Department of Functional
Morphology, Faculty of Veterinary Medicine, State University of Utrecht, Yalelaan 1, 3584 CL
0.M.B.) Utrecht, the Netherlands
ABSTRACT
A hitherto-unknown atavistic muscle in the dog initiated a review
of the literature on the homologies and nomenclature of the forelimb flexors in
carnivores and man. A consequence is that we recommend a revision of the nomenclature in the Nomina Anatomica Veterinaria (Ithaca, New York, 1983)so that it is
in agreement with the Nomina Anatomica (Wilkins, Baltimore, 1983). This revision
mainly consists of the incorporation of the terms M. palmaris longus and Mm.
flexores breves manus.
When studying the comparative myology of the forelimb and neck in Hyaenidae, Felidae, and Canidae
(Spoor and Badoux, 1986), various specimens of the dog
were dissected to gain a n impression of the variation in
topography and relative development of the muscles. In
a left canine forelimb of a mongrel dog of medium size,
we encountered a hitherto-undescribed head of the M.
flexor digitorum superficialis. The slender muscular
belly originates from the epimysium on the distal third
of the lateral aspect of the superficial digital flexor and
is innervated by the N. ulnaris. Its tendon terminates
in the fascia covering the accessory carpal bone (Fig. 1).
Since the specimen has a n M. flexor digitorum brevis in
a normal position, the above-mentioned belly should not
be seen as a proximal derivative of this muscle. In our
opinion the muscle is a n atavistic rather than a newly
evolved structure. In order to establish the correct name
and homologies of this muscle, we studied the myology
of the forelimb in various groups of carnivores (Cuvier,
1809; Meckel, 1828; Owen, 1868; Windle and Parson,
1897; Bronn, 1874-1900;Kajava, 1918; Grasse, 1971)and
arrived a t the conclusion that this muscle is the homologue of the M. palmaris longus ulnaris (s. internus) in
Mustelidae, Procyonidae, Ursidae, and Felidae. Since
this name is based on a n interpretation of the forearm
flexors of carnivores, which differs from that in veterinary anatomical nomenclature, we traced the background of the nomenclature of these muscles.
REVIEW AND DISCUSSION
The enumeration in the Nomina Anatomica Veterinaria (N.A.V., 1983) and the homologies mentioned in
current handbooks on veterinary anatomy are based on
the works of Gurlt (1860), Franck (18711, Ellenberger
and Baum (1891), Sussdorf(l895), and Martin (1904) and
on specialized studies on the homologies of the forearm
flexors by Aeby (1860), von Bardeleben (1890), Windle
(1889), Pitzorno (1905), and Kajava (1922, 1923). The
latter author has been incorrectly interpreted concerning the use of the name M. interflexorius (introduced by
Pitzorno, 1905) in carnivores. Eisler (18951, McMurrich
(1903), Howell (19361, Ribbing (19381, and Strauss (1942)
compared the forearm flexors of amphibians, reptiles,
0 1986 ALAN R. LISS, INC.
and (primitive) mammals to establish the homologies of
these muscles, which resulted in a phylogenetic model
(Strauss, 1942). This model confirms the comparative
interpretation of the forearm flexors in carnivores of
Kajava (1918). Unfortunately the studies by Strauss
(1942) and Kajava (1918) have been overlooked by authors of the handbooks on veterinary anatomy. Table 1
gives the names of the digital forearm flexors in Carnivora following the N.A.V. (19831, the Nomina Anatomica
(N.A., 1983) and authors which, in our view, describe
correctly the homologies between these muscles in
mammals.
Before commenting upon this scheme, some remarks
of a more fundamental nature should be made. It is
common practice to name muscles either on the basis of
homology (with muscles named either in earlier literature, or in editions of “Nomina”) or on the basis of their
function or shape. Application of the latter principles
leads inevitably to the designation of different names to
homologous muscles, especially in the case of man and
domestic mammals, as the following example will show.
In the 19th century, veterinary anatomists named the
M. palmaris longus in the dog M. flexor digitorum superficialis on the basis of a morphological characteristic,
i.e., its perforated terminal tendons. It must be emphasized, however, that the distal tendons and the proximal
muscular belly indeed form a functional entity but differ
fundamentally in their ontology. The perforated tendons
(including the manica flexoria) and the closely related
M. flexor brevis manus are the homologues of the Mm.
flexores breves superficiales in reptiles (Eisler, 1895;
Strauss, 1942). In mammals, the terminal tendons of the
(long) superficial digital flexor fuse with the perforated
tendons. In some mammalian species, the M. palmaris
longus is well- developed and its strong terminal tendons insert (as the main branch) on the individual manicae flexoriae of the digits.
The degree of development of the M. palmaris longus
and M. flexor digitorum superficialis varies more or less
inversely which is clearly demonstrated in species of
Carnivora. In Genetta and Herpestes (Kajava, 1918)as in
Received March 6,1986; accepted July 28,1986
472
C.F. SPOOR AND D.M. BADOUX
Fig. 1. a) Distal part of the left forelimb of a mongrel dog; palmar
view. ~ 0 . 6 3 b)
. Laterodorsal view of the superficial digital flexor.
x0.82. 1. M. flexor carpi ulnaris. 2. M. flexor digit. profundus. 3. M.
flexor digitorum superficialis. 4. Atavistic head, indicated by arrow. 5.
M. flexor digitorum brevis. 6. Accessory carpal bone. 7. Branch of the
N. ulnaris. (Names following the N.A.V.)
man, the superficial digital flexor is strongly developed
and originates by a very short tendon from the medial
humeral epicondyle; the M. palmaris longus is only
weakly developed and inserts in the palmar fascia. In
Canis, Panthera, Ursus, and Lutra (ibidem) the situation
is the reverse: the M. palmaris longus is strong and well
developed, while the superficial digital flexor is small.
The size and the origin from the deep digital flexor of
the latter muscle in the dog led veterinary anatomists
to designate i t as M. interflexorius, a name originally
reserved for the small additional connections between
the superficial and deep digital flexors (Pitzorno, 1905).
In Hyaena (Spoor and Badoux, 1986) the situation is
intermediate: the M. palmaris longus has relatively
weak and flat tendons; the bellies of the superficial
digital flexor originate from the medial humeral epicondyle by a long, flat tendon and from the deep digital
flexor. This variety in the development of the long digital forelimb flexors in carnivores represents the various
intermediate stages between the morphology in man
(named in the N.A.) and in the dog (named in the N.A.V.)
necessary to determine the correct homologies.
If the myological nomenclature should be based on
homologies of muscles, in the description of the dog and
cat the name M. interflexorius should be changed into
M. flexor digitorum superficialis and the name M. flexor
digitorum superficialis into M. palmaris longus. This
requires the incorporation of the term M. palmaris longus in the N.A.V. The initial subdivision of the M. palmaris longus (Kajava, 1918; Strauss, 1942)justifies the
designation of three separate muscles: the M. palmaris
longus ulnaris, -intermedius, and -radialis; the latter
rarely occurs in mammals. The M. palmaris longus ulnaris is innervated by the N. ulnaris and the M. palmaris longus intermedius either by the N. medianus or
by N. medianus and N. ulnaris (Agduhr, 1915; Strauss,
1942). Since it is not always clear how this subdivision
is represented in the morphology of the M. palmaris
longus of various mammals, it is preferable to distinguish a n M. palmaris longus with, if necessary, a caput
ulnare and a caput intermedius.
Since Mm. flexores breves manus are probably only
occurring in man in the form of some atavistic fibres in
the flexor retinaculum (Benninghoff, 1964), these muscles are not named in the N.A. The dog has only one of
these muscles (to digit V, named M. flexor digitorum
brevis in the N.A.V.), the cat two (to digits IV and V),
and some carnivores up to four. In order to designate
this muscle in the dog more accurately the general name
M. flexor digitorum brevis of the N.A.V. might be
changed into M. flexor digiti V brevis; however, this
name is already used in the N.A. for one of the deep
short muscles to digit V (Table 1). Instead of changing
this name in the N.A. we prefer the introduction of the
term of M. flexor brevis manus digiti V (or IV)into the
N.A.V., which is commonly used in the zoological literature. The name M. flexor digiti V in the N.A.V. should
be changed to M. flexor digiti V brevis.
Even without knowing exactly the phylogenetical
“background” of the mammal it is fairly easy to determine whether a muscle represents the M. palmaris longus or not, since its tendon has a fundamentally different
phylogenetical origin than those of the deeper digital
flexors. The tendon of the M. palmaris longus (caput
intermedius) is developed from subcutaneous mesenchyme (Strauss, 1942) and therefore passes palmar to
the flexor retinaculum. It is separated from the tendons
of the superficial and deep digital flexor (the interflexorius and deep digital flexor in the N.A.V.) and it is not
enclosed in the carpal synovial sheet. The tendon of the
M. palmaris longus caput ulnare usually inserts in the
473
NOMENCLATURE OF THE DIGITAL FORELIMB FLEXORS
TABLE 1. Nomenclature of some homologue forelimb flexors
Nomina Anatomica Veterinaria (1983)
(Carnivora, mainly Canis familiaris)
Nomina Anatomica (1983)
(Homo sapiens)
Kajava (1918);Strauss (1942)
(Carnivora)
M. palmaris longus
Not present
M. flexor digitorum superficialis
M. flexor digitorum brevis
M. palmaris longus
M. flexor brevis manus
M. flexor digitorum superficialis
M. interflexorius
M. flexor digitorum
M. flexor digitorum profundus
+ M. flexor pollicis longus
M. flexor digiti V brevis
M. flexor digitorum profundus
M. flexor digitorum
M. flexor digiti V
M. flexor digiti V brevis
(digiti 11-V)
sublimis
profundus
fascia covering the accessory carpal bone. As a consequence, the superficial belly of the M. flexor digitorum
superficialis (N.A.V., 1983) of artiodactyls is identical
with the M. palmaris longus (caput intermedius). The
interpretation of the digital forearm flexors of ungulates
is complicated by the reduction of the number of toes
and by the occurrence of various Mm. interflexorii (Kajava, 1923) and is not within the scope of this paper.
The Mm. flexores breves manus originate from the
palmar side of the flexor retinaculum, the fascia covering the accessory carpal bone or the M. palmaris longus.
In the cat the M. palmaris longus cap. ulnare is fused
with the Mm. flexores breves manus digiti N et V. This
complex was interpreted by Nickel et al. (1968) as the
M. flexor digitorum brevis (N.A.V.), by Mivart (1881)
and Getty (1975) as a n ulnar head of the M. flexor
digitorum superficialis, and by Barone (1967) (in the
lion) as the equivalent of the human M. palmaris longus.
Another example of the confusing consequences of two
noncompatible nomenclatures is found in Getty (19751,
who used the N.A.V. for the description of the forearm
flexors of the dog and the N.A. for that of the cat (whose
myological nomenclature historically follows that of
man).
The above leads us to advocate the opinion that myological nomenclature should be based on a phylogenetical rather than on a functional or morphological basis.
A consistent anatomical nomenclature is the best guaranty to prevent further confusion and misunderstanding
in research in comparative anatomy. We agree that this
point of view has some didactic disadvantages, but in
this case scientific importance should prevail. It should
be widely known that the medical and veterinary nomenclature are noncompatible.
The currently supposed compatibility is, e.g., shown
by the various attempts to trace the homologue of the
human M. palmaris longus and -brevis in the dog and
cat. Gurlt (18601, Ellenberger and Baum (18911, and
Barone (1980) considered the M. interflexorius (which is
the M. flexor digitorum superficialis) to be the homol o p e of the human M. palmaris longus and the M.
flexor brevis manus as the homologue of the M. palmaris
brevis. Sussdorf (1895) and Agduhr (1915) homologized
the ulnar head of the M. flexor digitorum profundus in
the dog with the M. palmaris longus.
We recommend a revision of the nomenclature of the
forearm flexors of carnivores as adopted in the N.A.V.
so that it is in agreement with the N.A., which is preferably used in zoological and human myology.
ACKNOWLEDGMENTS
We thank Mr. G. Hol and Mr. P. Hoogeveen for their
general assistance during the dissections and Mr. 0. van
der Veen for making the figures.
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