Anatomical studies on the thumb muscles of the human hand

The thenar muscles in 34 cases of 25 human adults were dissected using a stereoscopic microscope. The main supplying branch of the median nerve is often called the recurrent branch because of its characteristic recurrent course, and it enters the muscles between the abductor pollicis brevis and the superficial head of the flexor pollicis brevis. The other branch of the median nerve enters the latter muscle directly close to the tendon of the flexor pollicis longus laterally. This is called the direct branch, and was observed in 32 cases. Of these 32, it passed medial to the tendon in 7 cases. The deep terminal branch of the ulnar nerve supplies the transverse and the oblique heads of the adductor pollicis. One of the branches passes laterally piercing through the latter, and connects with the direct branch of the median nerve under the tendon of the flexor pollicis longus. The anastomosis between the two nerves was observed in 31 cases (91%). The obtained results concerning the nerve supply of the thenar muscles are summarized in Table 1. As seen in this table, the boundary of the supplying area between the median and ulnar nerves almost coincides with the position of the tendon of the flexor pollicis longus. In the other three cases in which a thick anastomosis, about 1 mm in thickness, existed between the median and ulnar nerves in the forearm (Martin-Gruber anastomosis), remarkable expansion of the supplying area of the ulnar nerve within the thenar muscles was observed, as seen in Table 2. According to the descriptions from clinical fields, the opponens pollicis and even the abductor pollicis brevis are sometimes supplied by the ulnar nerve. Such an extreme case may be explained by the existance of the anastomosis described above. The palmar interosseous muscle of the thumb has been described in textbooks. In this study, it is identified as the muscle fascicle of the adductor pollicis dorsal to the princeps pollicis artery, and was observed in 22 cases (65%). The macaca fuscata was also examined in 5 cases. In contrast to humans, all the thenar muscles can be identified without difficulty because no fusions are observed among them and because the nerve supply for each muscle is fairly constant.