Coracobrachialis muscle and the musculocutaneous nerve: a study using human embryonic sections

In comparative anatomy, the musculocutaneous nerve is hypothesized to pass between the superficial and deep muscle bellies of the coracobrachialis muscle. The superficial belly is supplied by nerve branches of the lateral cord of the brachial plexus, while the deep belly by the musculocutaneous nerve. Observations of longitudinal sections of ten human embryonic arms (7 weeks; crown-rump length 26-32 mm) demonstrated that the coracobrachialis muscle was always continuous with the short head of the biceps muscle. If the aforementioned hypothesis was applied, the deep belly behind the musculocutaneous nerve course was continuous with the biceps. However, such a close relation between the coracobrachialis and biceps was not known in supplying nerves in adults. A further study using embryos of some apes without the deep belly of the coracobrachialis would be necessary for the comparison between a pattern of the embryonic muscle division and the muscle classification in comparative anatomy.