[A morphological study on the coracobrachialis muscle].

The musculocutaneous nerve (MC) usually pierces the coracobrachialis muscle (cbr) in man. However in rare cases it does not pierce the muscle but runs downward adhering to the median nerve. Although this unusual course of MC is well noted in the literature, there has been no satisfactory explanation given for its derivation. The present author thought that cbr was a composite muscle innervated by at least two different nerves and a change in composition of the muscle parts might have something to do with the change in the course of MC. Therefore MC, cbr and its nerve supply were observed in 240 human arms. The nerve fiber analysis of MC and the branches to cbr were made in 27 arms, specially selected from them. Additionally, to correlate the results of humans and other mammals, the same analysis was performed using 4 prosimian arms (3 species). The results were as follows: 1. The nerve branches innervating human cbr could be classified into three groups. The first group branches were those constant in appearance, arising from MC and usually innervating the largest area of cbr. The author designated these branches Rmc (branches of MC). The second group consisted of a branch which arose from the middle trunk of the brachial plexus (the seventh cervical nerve), passed dorsal to MC and supplied the proximal deep part of cbr situated dorsal to the course of MC. This branch was also constant in appearance and called Rp (deep branch). The third group branched off from the ventral surface of the middle trunk (the seventh cervical nerve), passed ventral to MC and supplied the proximal ventral part of cbr situated ventral to the course of MC. This branch (called Rs, superficial branch) appeared in only 5 out of 27 cases. 2. The change in the course of MC was closely correlated with the change in the ratio of the part innervated by Rmc to the part innervated by Rp. In the case where the part innervated by Rmc was larger and the part innervated by Rp was smaller than usual, the ratio was larger than ordinary and MC pierced the deeper part of cbr. On the contrary, in the case where the part supplied by Rmc was smaller and the part supplied by Rp was larger than usual, the ratio was smaller and MC pierced the more superficial part. In extreme cases where the part supplied by Rmc was very slight, MC did not pierce cbr. 3. In prosimians, cbr was composed of two muscular parts.(ABSTRACT TRUNCATED AT 400 WORDS)