Supernumerary head of biceps brachii and branching pattern of the musculocutaneus nerve in Japanese.

Out of 546 upper limbs (273 cadavers), supernumerary heads of the biceps brachii were found in 75 limbs (13.7%) of 58 cadavers (21.3%). The form, origin, and insertion of the supernumerary heads, and branching pattern of the musculocutaneus nerve were studied. In addition, the dimensions of the heads were measured. In many cases, the supernumerary head arose from the humerus, between the insertion of the coracobrachialis and the upper part of the origin of the brachialis, and/or from the medial intermuscular septum. In a few cases, a supernumerary head arose from the tendon of the pectoralis major or the deltoid, or from the articular capsule, or from the crest of the greater tubercle. The supernumerary heads typically joined the common belly, or the aponeurosis of the biceps brachii. Some heads joined the belly of the long head or that of the short head. In the examination of the branching pattern of the musculocutaneus nerve, communication between the musculocutaneus nerve and the median nerve was found in 43 out of the 75 limbs (57.3%). The communicating branch ran from the musculocutaneus nerve to the median nerve in 24, from the median nerve to the musculocutaneus nerve in 12, in both directions in 5, or in another type of pattern in 2 out of 43 limbs. Sometimes a branch of the musculocutaneus nerve ran around a supernumerary head and then fused with the present trunk. The presence of a supernumerary head seemed to affect the course and branching of the musculocutaneus nerve.