Okajimas Folia Anat. Jpn., 66 (6): 339-354, March, 1990 Anatomical Study on the Extensor Digitorum Profundus Muscle in the Japanese By Yukio YOSHIDA Department of Anatomy, Kanazawa Medical University, Uchinada, Ishikawa 920-02, Japan - Received for Publication, July 25, 1989 Key words: Extensor digitorum profundus muscle, Extensor indicis muscle, Phylogeny, Variation, Type Summary: An anatomical study on the extensor digitorum profundus muscle was made using 832 upper limbs from 416 Japanese adults. The separate muscles derived from the extensor digitorum profundus consist of 10 kinds: namely, the extensor pollicis longus, extensor pollicis et indicis accessorius, extensor indicis radialis, extensor indicis proprius, extensor indicis ulnaris, extensor indicis et medii accessorius, extensor medii proprius, extensor annularis proprius, extensor carpi profundus and extensor digiti brevis. The configuration of the muscles (except for the extensor digiti brevis) in the upper limb was classified into 13 types according to their arrangement and insertion. The most frequent type involved coexistence of the extensor pollicis longus and the extensor indicis proprius: it was observed in 664 limbs (79.8%). The next type involved coexistence of the extensor pollicis longus, extensor indicis proprius and the extensor medii proprius: it was observed in 67 limbs (8.1%). It appears that the extensor digiti brevis of man is derived from the most ulnar part of the extensor digitorum profundus which does not migrate proximally. Many excellent papers have been published on the upper limb muscles of vertebrates. The extensor musculature of the forearm and hand is subject to great variability among primates including man. Its phylogenetic significance has been examined in research on the comparative anatomy. Investigations of the forearm extensor muscles have been made by Ribbing (1938), Straus (1941) in many vertebrates, Yoshida (1978), Yoshida and Fukuyama (1979), Kosugi et al. (1984a), Yoshida (1985), and Aziz and Dunlap (1986) in man. However, there have been few reports concerning the extensor pollicis longus and extensor indicis (extensor digitorum profundus muscle) based on man. Therefore, the frequency of occurrence, and the morphology of each variant muscle in man have remained unknown. The present paper is concerned with the extensor digitorum profundus muscle (including the extensor digiti brevis) of man. Materials and Methods The present data were based on the gross anatomical dissection of 832 upper limbs from 416 adult cadavers (217 Japanese males and 199 Japanese females). The materials were examined at the Department of Anatomy, School of Medicine, Chiba University, and 339
340 Y. Yoshida Kanazawa Medical University, during 1973 to 1987. Observations were made on the extensor pollicis longus, extensor indicis and their supernumerary muscles. The origin and insertion of each muscle were examined from the 832 upper limbs forming the series. Particular attention was paid to the points where the tendons of the supernumerary muscles were attached. Observations The extensor digitorum profundus exhibits many types of division. The following 10 separate muscles were noted in this study: A. Extensor pollicis longus B. Extensor pollicis et indicis accessorius C. Extensor indicis radialis D. Extensor indicis proprius E. Extensor indicis ulnaris F. Extensor indicis et medii accessorius G. Extensor medii proprius H. Extensor annularis proprius I. Extensor carpi profundus J. Extensor digiti brevis. All of these muscles were innervated by the posterior interosseus branch of the radial nerve. A. Extensor pollicis longus. This muscle is the most radial component of the extensor digitorum profundus. It originates principally from the dorsal aspects of the ulna and interosseus membrane radial and proximal to the origin of the extensor indicis proprius, and inserts dorsally on the phalanges of the thumb. The extensor pollicis longus was constantly found in all 832 upper limbs. The muscle belly was separated in 827 cases, and fused with that of the extensor indicis proprius in 4 and with that of the extensor pollicis et indicis accessorius in 1 case. Its tendon was single in 802 and duplicated in 30 cases. Neither duplication nor absence of the muscle belly was present. B. Extensor pollicis et indicis accessorius. This muscle is seen between the extensor pollicis longus and the extensor indicis proprius. Its single tendon bifurcates in the hand to supply the thumb and the index finger. The radial portion of the tendon is usually united with the extensor pollicis longus tendon at the distal part, and the ulnar portion is attached to the radial dorsum of the index finger. The extensor pollicis et indicis accessorius was present in 10 upper limbs (1.2%). The muscle belly was separated in 8 cases. It was fused with the muscle belly of the extensor pollicis longus in 1 case and with that of the extensor indicis proprius in 1 case. C. Extensor indicis radialis. This muscle arises from the dorsal aspects of the ulna and the neighboring interosseus membrane. It lies between the extensor pollicis longus and the extensor indicis proprius, and inserts on the radial side of the index finger. The extensor indicis radialis was present in 24 upper limbs (2.9070. The muscle belly was separated in 5 cases, and fused with that of the extensor indicis proprius in 19 cases. D. Extensor indicis proprius. This muscle takes its origin primarily from the dorsal aspect of the ulna medial and below the extensor pollicis longus, from the interosseus membrane and from the extensor pollicis longus septum. Its single tendon, passing under the extensor retinaculum in company with the tendons of the extensor digitorum communis, is attached to the extensor Fig. 1. Various types of the extensor digitorum profundus in man. Abbreviations: A, extensor pollicis longus; B, extensor pollicis et indicis accessorius; C, extensor indicis radialis; D, extensor indicis proprius; E, extensor indicis ulnaris; F, extensor indicis et medii accessorius; G, extensor medii proprius; H, extensor annularis proprius; I, extensor carpi profundus; J, extensor digiti brevis; 2, index finger; 3, middle finger; 4, ring finger.
Extensor Digitorum Profundus Muscle in Japanese 341
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Extensor Digitorum Profundus Muscle in Japanese 343 aponeurosis and phalanges of the index finger ulnar to the corresponding tendon of the extensor digitorum communis. The extensor indicis proprius was present in 810 upper limbs (97.407o). Its tendon was duplicated in 9 cases. The muscle belly was separated in 729 cases, but small in 10 cases. The muscle belly was fused with that of an adjoining muscle in 81 cases (4 with the extensor pollicis longus, 1 with the extensor pollicis et indicis accessorius, 19 with the extensor indicis radialis, 18 with the extensor indicis ulnaris, 8 with the extensor indicis et medii accessorius, 28 with the extensor medii proprius, and 3 with the extensor carpi profundus). E. Extensor indicis ulnaris. This muscle is seen medial to the extensor indicis proprius. Its tendon is attached more ulnar to the extensor indicis proprius tendon on the index finger. The extensor indicis ulnaris was present in 24 upper limbs (2.9%). The tendon was duplicated in 1 case. The muscle belly was separated in 3 cases. The muscle belly was fused with that of the extensor indicis proprius in 16 and with that of the extensor medii proprius in 3 cases. In 2 cases, the muscle belly was divided into two parts. The radial part was fused with the muscle belly of the extensor indicis proprius, and the ulnar part with that of the extensor medii proprius. The fine tendons from both parts were united to form a single tendon which was attached to the index finger more ulnar to the extensor indicis proprius tendon. F. Extensor indicis et medii accessorius. This muscle is coexistent with the extensor pollicis longus and extensor indicis proprius. It arises from the dorsal aspect of the ulna medial to the extensor indicis proprius. Its single tendon bifurcates at the distal portion to supply the more ulnar side of the index fmger and the radial side of the middle finger. There is frequently a membranous connection between the bifurcated branches. The extensor indicis et medii accessorius was present in 12 upper limbs (1.4%). The muscle belly was separated in 4 cases, and fused with that of the extensor indicis proprius in 8 cases. G. Extensor medii proprius. This muscle is seen with the extensor pollicis longus and extensor indicis proprius. It originates from the dorsal aspect of the ulna, medial and distal to the latter or the extensor indicis ulnaris (if this is present). Its single tendon passes deep to the extensor retinaculum and ends on the ulnar side of the middle finger. The extensor medii proprius was present in 80 upper limbs (9.6%). The muscle belly was separated in 47 cases. It was fused with the muscle belly of the extensor indicis proprius in 28 and with that of the extensor indicis ulnaris in 5 cases. H. Extensor annularis proprius. This muscle is small and coexistent with the extensor pollicis longus, extensor indicis proprius and extensor medii proprius. It arises from the dorsal aspect of the ulna medial to the extensor medii proprius and is attached to the ulnar side of the ring finger. The extensor annularis proprius was present in 1 upper limb (0.1 7o). Its tendon was duplicated. I. Extensor carpi profundus. This muscle is small and seen medial to the extensor indicis proprius. It inserts on the dorsum of the middle carpus. The extensor carpi profundus was present in 9 upper limbs (1.107o). The muscle belly was separated in 6 cases, and fused with that of the extensor indicis proprius in 3 cases. J. Extensor digiti brevis. This muscle usually originates from the distal margin of the radius and/or the joint capsules overlying the carpus beneath the cover of the extensor digitorum communis. It commonly extends distally as one muscle fascicle in man. Its tendon ends on the ulnar side of the index or middle finger, or is united with the adjoining
344 Y. Yoshida one at the distal portion. The extensor digiti brevis was present in 11 out of 22 upper limbs which did not have the extensor indicis proprius (50.007o, subtype 13a). The insertion of the muscle was the same as that of the extensor indicis proprius. The extensor digiti brevis was present in only 7 out of 810 upper limbs which had the extensor indicis proprius (0.9070. It was coexistent with the extensor indicis proprius in 5 cases (subtype 12a). Each extensor digiti brevis tendon was united with the extensor indicis proprius tendon at the distal portion in 4 cases, and with the extensor digitorum communis tendon for the middle finger in 1 case. The extensor digiti brevis was seen with the extensor indicis proprius and the extensor pollicis et indicis accessorius in I. case (subtype 2a). Its tendon was united with that of the extensor indicis proprius at the distal portion. The extensor digiti brevis was seen with the extensor indicis et medii communis in 1 case (subtype lob). Its tendon was attached ulnar to the medial tendon of the extensor indicis et medii communis on the middle finger. The configurations of the muscles derived from the extensor digitorum profundus (except for the extensor digiti brevis) were as follows. The abbreviations (A to I) are as indicated above. Four muscles (A, B, D and G) were present in 2 upper limbs (type 1). They were separated respectively. Three muscles (A, B and D) were present in 8 limbs (type 2). They were separated respectively in 6 cases (subtypes 2a and 2b). B and D were fused together in 1 case (subtype 2c), and A and B were fused together in 1 case (subtype 2d). Four muscles (A, C, D and G) were present in 1 limb (type 3). C and D were fused together. Three muscles (A, C and D) were present in 23 limbs (type 4). They were separated respectively in 5 cases (subtype 4a). C and D were fused together in 18 cases (subtype 4b). Four muscles (A, D, E and G) were present in 9 limbs (type 5). They were separated respectively in 1 case (subtype 5a). E and G were fused together in 3 cases (subtype 5b), and D and E were fused together in 3 cases (subtype 5c). The extensor indicis ulnaris (E) was divided into two parts in 2 cases (subtype 5d). The radial part was fused with D, and the ulnar one with G. Three muscles (A, D and E) were present in 15 limbs (type 6). They were separated respectively in 2 cases (subtype 6a). D and E were fused together in 13 cases (subtype 6b). Four muscles (A, D, F and 1) were present in 1 limb (type 7). They were separated respectively. Three muscles (A, D and F) were present in 11 limbs (type 8). They were separated respectively in 3 cases (subtype 8a). D and F were fused together in 8 cases (subtype 8b). Four muscles (A, D, 0 and H) were present in 1 limb (type 9). They were separated respectively. Three muscles (A, D and G) were present in 67 limbs (type 10). They were separated respectively in 39 cases (subtype 10a). D and G were fused together in 28 cases (subtypes lob and 10c). Three muscles (A, D and I) were present in 8 limbs (type 11). They were separated respectively in 5 cases (subtype 11a). D and I were fused together in 3 cases (subtype 11b). Two muscles (A and D) were present in 664 limbs (type 12). They were separated respectively in 660 cases (subtypes 12a and 12b), and fused together in 4 cases (subtype 12c). The extensor indicis proprius (D) was completely absent and the extensor pollicis longus (A) only was present in 22 limbs (type 13). There were 11 forms of fusion of the muscles derived from the extensor digitorum profundus (except for the extensor digiti brevis). Fusion of A and B was present in 1 case,
Extensor Digitorum Profundus Muscle in Japanese 345 and fusion of A and D in 4 cases. Fusion of B and D was present in 1 case. Fusion of C and D was present in 19 cases. Fusion of D and E was present in 16, and fusion of D and the radial part of E in 2 cases. Fusions of D and F, of D and G, and of the D and I were present in 8, 28, and 3 cases, respectively. Fusion of E and 0 was present in 3, and fusion of the ulnar part of E with G in 2 cases. Discussion In order to understand the varieties of the extensor indicis and extensor pollicis longus (extensor digitorum profundus), a detailed knowledge of the phylogeny of the forearm and hand muscles is essential. The extensor musculature of the forearm and hand in quadrupeds involves three groups: the brachio-antebrachial, antebrachio-manual, and manual (Straus, 1941). The brachioantebrachial group basically takes its origin from the dorsal aspect of the brachial bone and inserts on the antebrachial bones. It acts mainly on the elbow joint. The antebrachiomanual group basically takes its origin from the dorsal aspects of the antebrachial bones (including the interosseus membrane) and inserts on the manual bones. It acts mainly on the wrist joint. The manual group takes its origin from the dorsal aspect of the carpus and/or metacarpus and inserts on the digits (thumb and fingers). It acts mainly on the joints of the thumb and fingers. With the evolution of species, the brachioantebrachial group has extended its insertion to the distal portion of the upper limb. On the other hand, the origin of the antebrachiomanual group has undergone proximal migration during the phylogenetic course (Ribbing, 1938; Straus, 1941). The brachio-antebrachial group has been referred to as the superficial layer of the forearm extensor musculature, and the antebrachio-manual group as its deep layer in mammals. The antebrachio-manual group has undergone a extensive phylogenetic differentiation into separate muscles. The antebrachio-manual group of amphibians gives rise to the spinator manus and the extensor digitorum brevis. The former is represented by the abductor pollicis longus (wide sense), and the latter by the extensor digitorum profundus in basic mammals. The form and extent of the antebrachio-manual group in mammals appears to be highly variable. The abductor pollicis longus (wide sense) develops further into the abductor pollicis longus (narrow sense) and the extensor pollicis brevis, while the extensor digitorum profundus develops into the extensor pollicis longus and the extensor indicis proprius in man. The extensor pollicis longus has been referred to as the first part of the extensor digitorum profundus, and the other muscles (extensor indicis proprius and supernumerary muscles in man) as its second part (Straus, 1941). The extensor pollicis longus is the most radial component of the extensor digitorum profundus. In the current series, the first part (extensor pollicis longus) exhibited a marked stability, while the second part appeared to be highly unstable. The following description of the author's data therefore emphasizes the second part. There were seven supernumerary muscles (except for the extensor digiti brevis) which belonged to the second part. It or they were always coexistent with the extensor indicis proprius and the extensor pollicis longus on the upper limb. The extensor pollicis et indicis accessorius and the extensor indicis radians lie radial to the extensor indicis proprius. On the other hand, the extensor indicis ulnaris, extensor indicis et medii accessorius, extensor medii proprius, extensor annularis proprius and the extensor carpi profundus lie ulnar to it. Varieties in the reported cases of the extensor
346 Y. Yoshida digitorum profundus are shown in Figure 1. When cleavage of the extensor digitorum profundus into the extensor pollicis longus and extensor indicis proprius does not occur, the radial component of it forms the extensor pollicis et indicis communis. It is attached to the pollex (thumb) and the second digit (index finger). However, the extensor pollicis et indicis accessorius as named by the author is coexistent with the normal extensor pollicis longus and extensor indicis proprius, lying between these two muscles. This does not therefore correspond correctly to the extensor pollicis et indicis communis of mammals. It is important to emphasize that this anomalous muscle of man is a supernumerary one. In the present study, the extensor pollicis et indicis accessorius was seen in 10 out of 832 upper limbs (1.2%). It showed a tendency to separate from the extensor pollicis longus and extensor indicis proprius. The separated anomalous muscle passed below the extensor retinaculum adjacent with the extensor indicis proprius in 8 cases. The muscle was fused with the extensor pollicis longus in 1 case, and with the extensor indicis proprius in 1 case. In the former case, the tendon of the extensor pollicis et indicis accessorius passed adjacent with that of the extensor pollicis longus. Coexistence of the extensor pollicis et indicis accessorius with the extensor pollicis longus and extensor indicis proprius in an upper limb was classified into type 2, and coexistence of it with these muscles and the extensor medii proprius was classified into type 1. The radial portion of the tendon of the extensor pollicis et indicis accessorius is smaller than the extensor pollicis longus tendon, and the ulnar portion is smaller than the extensor indicis proprius tendon. This anomalous muscle may therefore act little on the joints of the thumb and index finger in man. The two portions of the tendon of each extensor pollicis et indicis accessorius are variable in size. Hence, if the radial portion is reduced and lost, the muscle may change into the extensor indicis radialis. Coexistence of the extensor indicis radialis with the extensor pollicis longus and extensor indicis proprius in an upper limb was classified into type 4, and coexistence of it with these muscles and the extensor medii proprius was classified into type 3. The extensor indicis ulnaris lies medial to the extensor indicis proprius and is thinner than it. Coexistence of the extensor indicis ulnaris with the extensor pollicis longus and extensor indicis proprius in an upper limb was classified into type 6, and coexistence of it with these muscles and the extensor medii proprius was classified into type 5. There have been few reports on the extensor indicis et medii accessorius. The distal part of its tendon frequently looks membranous. This anomalous muscle did not coexist with the extensor indicis ulnaris or the extensor medii proprius. It seems therefore that the extensor indicis et medii accessorius is an intermediate form of the extensor indicis ulnaris and the extensor medii proprius. Coexistence of the extensor indicis et medii accessorius with the extensor pollicis longus and extensor indicis proprius in an upper limb was classified into type 8, and coexistence of it with these muscles and the extensor carpi profundus was classified into type 7. Coexistence of the extensor medii proprius with the extensor pollicis longus and extensor indicis proprius in an upper limb was classified into type 10. Coexistence of the extensor annularis proprius with the above three muscles was classified into type 9. A tendon of the second part for the ring finger (extensor annularis proprius) is very rarely found in man, but it is commonly demonstrated in Old World monkeys (Straus, 1941). Several other cases of this anomalous muscle in man have been reported by
Extensor Digitorum Profundus Muscle in Japanese 347 Cauldwell et al. (1943) and Aziz and Dunlap (1986). In these cases, the muscle bellies of the extensor annularis proprius and the extensor medii proprius were fused together. This combination has been designated as the extensor medii et annularis communis. There have been few reports on the extensor carpi profundus. In order to distinguish it from the extensores carpi radialis longus and brevis and the extensor carpi ulnaris, this anomalous muscle is termed the extensor carpi profundus by the author. When a small supernumerary muscle develops medial to the extensor indicis proprius, its fine tendon cannot extend as far distally as the finger and may end on the carpus. Coexistence of the extensor carpi profundus with the extensor pollicis longus and extensor indicis proprius in an upper limb was classified into type 11. The separate muscles derived from the extensor digitorum profundus were found to be variably inserted on the thumb, the index to ring fingers and on the carpus in the present study. During the phylogenetic course of the extensor digitorum profundus, its ulnar component gradually shows regression. The extensor digiti profundus basically supplies the first to third or fourth digits (thumb to middle or ring finger) in many primates. It develops into the extensor pollicis longus and the extensor indicis proprius in man. It seems reasonable to suggest that they control the independent movement of the thumb and the index finger, respectively. This basic form (type 12) was observed in 664 upper limbs (79.8070 in the current series. For the above-mentioned phylogenetic reasons, the extensor digitorum profundus, especially the second part, exhibits many types of variation. There was a supernumerary muscle or muscles (except for the extensor digiti brevis) in 146 limbs (17.6%) in the present study. The most frequent extra muscle was the extensor medii proprius. It was present in 80 upper limbs (9.6%). The next muscles were the extensor indicis radialis and the extensor indicis ulnaris. They were observed in 24 limbs (2.9%) each. The fusion of the muscles derived from the extensor digitorum profundus (except for the extensor digiti brevis) revealed 11 forms. The most frequent form was a combination of the extensor indicis proprius and the extensor medii proprius (28 cases). This combination has been described as the extensor indicis et medii communis. The association of two tendons with the index and middle fingers is the commonest pattern of the second part in primates (Straus, 1941). In types 3 and 4, the extensor indicis radialis was fused with the extensor indicis proprius in 19 out of 24 cases. In types 5 and 6, the extensor indicis ulnaris was fused with the extensor indicis proprius in 18 out of 24 cases. Combination of each anomalous muscle with the extensor indicis proprius is usual. These combinations have been termed duplicated tendons of the extensor indicis. In such cases, the insertions of the two tendons should be carefully observed. The extra tendon which is attached radial to the corresponding extensor digitorum communis tendon on the index finger is derived from the extensor indicis radialis, and the one which is attached medial to the extensor indicis proprius tendon is derived from the extensor indicis ulnaris. If the muscle belly of the extensor indicis radialis or the extensor indicis ulnaris is separated from that of the extensor indicis proprius in an upper limb, these two muscles have been termed a doubled extensor indicis. Regression of the muscle complex like the second part of the extensor digitorum profundus is seen in the extensor digiti minimi (Kaneff, 1959a, b; Kaneff and Cihak, 1970; Yoshida, 1985). In mammals, the extensor digitorum lateralis (extensor digitorum quinti et quarti) usually inserts on the third or fourth to fifty digits (middle or ring to little fingers).
348 Y. Yoshida With specialization, however, its radial component may exhibit a tendency towards suppression. The muscle which has a single or double tendons for the ulnar side of the little finger is the commonest form in man, and it becomes the extensor digiti minimi. The extensor digiti minimi of man makes it possible for the little finger to move independently. Related to this phylogenetic evolution, the extensor digiti minimi frequently sends an extra tendon to the ring finger or the carpus in man. The manual group, which arises from the dorsum of the carpus and/or metacarpus and inserts on the digits (fingers), is commonly observed in amphibians and reptiles. Extension of the digits in them is primarily controlled by the manual group. During the evolution of the forearm and hand, especially the fingers, however, its function has been taken over by the antebrachio-manual and brachio-antebrachial groups, and the manual group has disappeared. Muscles which are similar to the manual group are absent in almost all mammals except man. Souter (1966) stated that the second part of the extensor digitorum profundus (deep layer of the forearm extensor musculature) may potentially be derived from a combination of the antebrachio-manual and manual groups. The extensor digiti brevis is a well-known anomaly in man. In the current series, absence of the extensor indicis proprius in an upper limb was classified into type 13. It was noted in 22 upper limbs, but the extensor digiti brevis was compensatorily present in 11 out of these cases (subtype 13a). On the other hand, it was found in only 7 out of 810 limbs which had the extensor indicis proprius. In 6 limbs (subtypes 2a and 12a) of the latter, the tendons of the anomalous muscle and extensor indicis proprius were fused together at the distal portion. The extensor digiti brevis thus shows a tendency to develop in an upper limb which does not have the extensor indicis proprius or to fuse with it at the distal portion. Moreover, there are cases of coexistence of the extensor digiti brevis with the supernumerary muscle of the second part in an upper limb. Kosugi et al. (1984b) described the coexistence of the extensor digiti brevis with the extensor indicis et medii communis (2 cases), and with the extensor medii proprius (1 case). Its coexistence with the extensor indicis et medii communis was observed in 1 limb (subtype 10b) in the present study. In such cases, the tendon of the extensor digiti brevis was fused with the tendon of the supernumerary muscle or attached medial to it on the middle finger. The innervation of the extensor digiti brevis and extensor indicis proprius has been found to involve the posterior interosseus branch of the radial nerve (Yoshida et al., 1984). However, the dorsal interosseus muscles which are seen on the dorsal hand are supplied by the deep branch of the ulnar nerve. It has been considered therefore that the dorsal interosseus muscles belong to the intrinsic muscles of the palm. The extensor digiti brevis of man is thus thought to be a variant of the extensor indicis proprius. It may be derived from the ulnar part or whole of the second part of the extensor digitorum profundus which does not migrate proximally. References 1) Aziz, M.A. and Dunlap, S.S.: The human extensor digitorum profundus muscle with comments on the evolution of the primate hand. Primates, 27: 293-319 (1986). 2) Cauldwell, E.W., Anson, B.J. and Wright, R.R.: The extensor indicis proprius muscle. Q. Bull. Northwest. Univ. Med. Sch., 17: 267-279 (1943). 3) Kaneff, A.: Beitrag zur Morphologie und Ausbildung des M. ext. digitorum lateralis bei einigen fiinffingrigen Sdugern (Felis domestica, Canis familiaris, Orictolagus cuniculus und Rattus rattus). Gegenbaurs Morphol. J15., 100: 115-130 (1959a).
Extensor Digitorum Profundus Muscle in Japanese 349 4) Kaneff, A.: Zur Frage der Ausbildung des Kleinfinger streckers, M. extensor digiti minimi, beim Menschen. Anat. Anz., 107: 365-374 (1959b). 5) Kaneff, A. and Cihak, R.: Die Umbildung des M. extensor digitorum lateralis in der Phylogenese und in der menschlichen Ontogenese. Acta Anat., 77: 583-604 (1970). 6) Kosugi, K., Meguri, S., Fukushima, 0. and Koda, M.: Anatomical study on the variation of the extensor muscles. 1. M. extensor indicis. Tokyo Jikeikai Med. J., 99: 867-876 (1984a) (in Japanese). 7) Kosugi, K., Fujishima, A., Koda, M. and Tokudome, M.: Anatomical study on the variation of the extensor muscles of the forearm. 2. M. extensor digitorum manus brevis. Tokyo Jikeikai Med. J., 99: 877-883 (1984b) (in Japanese). 8) Ribbing, L.: Die Muskeln und Nerven der Extremitaten. In: Bolk, L. et al. (eds.) Handb. vergleich. Anat. Wirbeltiere. Bd. 5, 543-656, Urban, Berlin (1938). 9) Souter, W.A.: The extensor digitorum brevis manus. Brit. J. Surg., 53: 821-823 (1966). 10) Straus, W.L.: The phylogeny of the human forearm extensors. Hum. Biol., 13: 23-50, 203-238 (1941). 11) Yoshida, Y.: Studies on the extensor indicis muscle in man. Acta Anat. Nippon., 53: 1-15 (1978) (in Japanese). 12) Yoshida, Y.: A study on the extensor digiti minimi muscle in man. Acta Anat. Nippon., 60: 185-196 (1985) (in Japanese). 13) Yoshida, Y. and Fukuyama, U.: Phylogeny of forearm extensor muscles in tetrapods. J. Kanazawa Med. Univ., 4: 122-130 (1979) (in Japanese). 14) Yoshida, Y., Yasutaka, S. and Seki, Y.: A study on the extensor digitorum brevis manus muscle in man. Acta Anat. Nippon., 59: 313-321 (1984) (in Japanese).
350 Y. Yoshida Explanation of Figures Plate I Fig. 2. Type 2. The extensor pollicis et indicis accessorius is seen. Its tendon bifurcates at the hand to supply the thumb and the index finger. Fig. 3. Type 4. The extensor indicis radians is present. Its tendon is attached to the radial side of the index finger. Fig. 4. Type 5. The extensor indicis ulnaris and the extensor medii proprius are seen. The former inserts on the more ulnar side of the index finger, and the latter on the ulnar side of the middle finger. Fig. 5. Type 7. The extensor indicis et medii accessorius and the extensor carpi profundus are present. The former is attached to the more ulnar side of the index finger and the radial side of the middle finger, and the latter to the middle carpus.
Extensor Digitorum Profundus Muscle in Japanese 351 Plate I
352 Y. Yoshida Plate II Fig. 6. Type 8. The extensor indicis et medii accessoris is seen. Fig. 7. Type 9. The extensor medii proprius and the extensor annularis proprius are present. The latter has double tendons which are inserted on the ring finger. Fig. 8. Type 10. The second part of the extensor digitorum profundus consists of the extensor indicis proprius and the extensor medii proprius. Fig. 9. Type 13. The extensor indicis proprius is completely absent, but the extensor digiti brevis is compensatorily present. Its tendon ends on the ulnar side of the index finger.
Extensor Digitorum Profundus Muscle in Japanese 353 Plate II