A morphological study on the coracobrachialis muscle
Koizumi, M.
Kaibogaku Zasshi. Journal of Anatomy 64(1): 18-35
1989
ISSN/ISBN: 0022-7722 PMID: 2508409 Document Number: 344623
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 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 the 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. (1) In comparison of the innervation pattern in prosimians and that in man, it is reasonable to assume that the coracobrachialis profundus muscle (cbrp, origin: the coracoid process, insertion: the surgical neck of the humerus) in prosimians is homologous with the muscular part innervated by Rp in man. And (2) the coracobrachialis superficialis muscle (cbrs, origin: coracoid process, insertion: a medial area of the humerus from its middle third to the medial epidondyle) in prosimians is homologous with th part innervated by Rmc in man. 4. In traditional studies of comparative anatomy, cbr has been described as being composed of three muscular parts, i.e. coracobrachialis brevis, medius and longus or coracobrachialis profundus, medius and superficialis. In the results of this study of fiber analysis of the supplying nerve, it is revealed that there is no firm ground for the distinction between the coracobrachialis medius and the coracobrachialis longus (or superficialis) and that the author's classification of two muscular parts, i.e. coracobrachialis profundus innervated by the nerve homologous with human Rp and coracobrachialis superficialis by the nerve homologous with human Rmc, seems reasonable. 5. The propriety of distinction between the muscular part innervatd by Rs and the part innervated by Rmc has not yet been justified and is crucial to solve. 6. Some aberrant muscles were observed around cbr in man. They could be divided into two groups. The first group consisted of an aberrant muscle which was found deep to the normal cbr and innervated by a nerve from the ventral surface of the posterior cord. The author concludes thtat this aberrant muscle is not the coracobrachialis profundus as others have reported. The second group consisted of three muscular slips wich were separated from cbr or the short head of the biceps brachii muscle, ran superficial to the median nerve and the brachial artery, and attached to the medial intermuscular septum. The findings on the nerve supply revealed that these three aberrant muscular slips did not have the same origin and one was a part of the short head of the biceps and the other two derived from the muscular part of cbr innervated by Rmc. Thus these three aberrant muscular slips are suggested to belong to their original muscles. 7. From the present study, it is clear that the human cbr is also composed of two different muscles; cbrs in which the insertion is more proximal in man than in prosimians,and cbrp which adheres to the deep surface of cbrs. It has also been clarified that MC proceeds proximally betw een cbrs and cbrp and distally pierces cbrs.