Functionally unimportant the muscle nevertheless shows several interesting morphological and anatomical characteristics. It is the counterpart in the arm of the adductors (longus, brevis, magnus) of the thigh. It arises from the apex of the coracoid process, where it is fused with the medial side of the short head of biceps. The tendon is continued into a muscular belly of varying development which is inserted into the medial border of the humerus. The lower extent of the insertion is marked by the nutrient foramen of the humerus, for the nutrient branch of the brachial artery runs along the lower border of the muscle. The upward extent of the insertion cannot be seen on most bones, the muscle usually leaving no impression.
The musculocutaneous nerve passes through the muscle. This is explained in terms of comparative anatomy. In some animals the muscle has three heads; in man two have fused, trapping the nerve between them, and the third part has become suppressed. The occasional supratrochlear spur (on the anteromedial aspect of the lower humerus) may be continuous with a ligament (of Struthers, Fig. 2.21) which passes to the medial epicon- dyle and represents the remains of the third head. The median nerve or brachial artery or both may run beneath it and be subject to compression.
Nerve supply of coracobrachialis:
By the musculocutaneous nerve (C5 and 6). The main trunk of the nerve passes through the muscle, first giving off its branch of supply.
Action of coracobrachialis:
Compared to the morphological interest of this muscle its action is negligible. It is a weak adductor of the shoulder joint, the main adductors of which are pectoralis major and latissimus dorsi.