High incidence of the third head of biceps brachii in South African populations.

Three different origins were found for the third head of the biceps brachii: (1) an origin from the humeral shaft inferior to and in common with the insertion area for the coracobrachialis; (2) a brachial origin in which the muscle originated distally from the medial humeral shaft, adjacent to and in common with the brachialis muscle; (3) a dual origin in which the medial fibres originated from the short head of biceps and the lateral fibres from the deltoid fascia and the insertion area of this muscle. In all cases, the third head inserted together with the other 2 heads of biceps brachii into the bicipital aponeurosis and the radial tuberosity. The third head receives its innervation from the musculocutaneous nerve, as do the other 2 heads of biceps. When there is a dual origin, partial innervation from the axillary nerve is possible since the fibres of the third head intermingle with those of the deltoid. There was a 20.5% incidence of the third head in South African blacks and only an 8.3% incidence in South African whites. Statistical analysis showed the difference between South African whites and blacks to be significant (P < 0.05). Not all of the blacks, however, had a higher incidence, since the Zulus showed a significant absence (P < 0.05) of the third head when compared with the other black populations.