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. LITERATURE CITED Aeby, C.T. (1860) Die Muskeln des Vorderarmes und der Hand bei Saugetieren und beim Menschen. Z. wissenschaftlichen Zool., 10t34-87. Agduhr, E. 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