Nerve fibre tracing of branches to the coracobrachialis muscle in a Bornean orangutan (Pongo pygmaeus pygmaeus).

A detailed anatomical analysis of the left brachial plexus, composed of the fourth cervical to first thoracic spinal nerve roots, was performed in an adult male orangutan obtained from the Osaka Museum of Natural History. Although the medial and posterior cords fused into a common trunk, a nerve fibre analysis revealed that the cords were not actually connected. A superficial branch (Rs) running ventral to the musculocutaneous nerve (MC) and a deep branch (Rp) running dorsal to the MC innervated the coracobrachialis muscle, which was also innervated by coracobrachialis branches (Rmc) from the MC. Koizumi (1989: Acta Anat. Nippon, 64, 18) reported that the Rmc and Rs innervated the superficial region of the coracobrachialis muscle corresponding to superficial coracobrachialis muscle in prosimians, whereas the Rp innervated the deep region. However, the detailed innervation of the coracobrachialis muscle in orangutans was not included in Koizumi and Sakai's (1995: J. Anat. 186, 395) report. Our observations in an orangutan did not seem to support generalisation, because the common trunk of the Rp and Rmc appeared to innervate both the superficial and deep regions of the coracobrachialis muscle. Therefore, a nerve fibre analysis of the common trunk was performed as a detailed examination. The analysis confirmed that the Rp in the orangutan only innervated the deep region of the coracobrachialis muscle, similar to the innervation pattern seen in other apes and humans.