Revisiting the anatomy of the pectoral nerves and nerve loops of the brachial plexus in the goat (Capra hircus)

Background: The anatomy of the pectoral nerves and the two nerve loops on the course of the axillary artery was revisited to complement current general descriptions as well as to argue whether the nerves contributing to the formation of the pectoral loop are the cranial pectoral nerves. Besides, the positional relationship between the scalene muscles and the nerve roots of the brachial plexus, which contribute to the nerves aimed in this study, was also examined at the same time as the dissection.

Methods: Twenty brachial plexuses of 10 domestic adult goats (8 females and 2 males) were examined using gross dissection in this study.

Results: In many dissections (95 %), the last bundle of scalenus ventralis muscle was found to pass between the roots of C7 and C8, dividing the brachial plexus into the cranial (ventral) and caudal (dorsal) parts. Two pectoral nerves were noted to contribute to the formation of the first nerve loop around the axillary artery. The first pectoral nerve arose predominantly from the ventral branches of C6 and C7 in company with the n. musculocutaneus, while the second pectoral nerve arose directly from C8 in 70 % of the dissections or as the first branch of the n. thoracicus lateralis (C8, T1) in the remaining 30 %. After the nerve loop, the second pectoral nerve branched off to innervate the superficial surface of the m. pectoralis profundus toward its insertion. The m. subclavius was found to receive its innervation from several sources, including the ventral branches of the brachial plexus. Interestingly, in 4 of the 14 dissections a communication between the n. subclavius and the n. phrenicus heretofore not found in the animal anatomy literature was found. In 16 of the 20 dissections (60 %), the ramus muscularis proximalis of the n. musculocutaneus received the communicating branch(s) from the n. medianus at the site of the second nerve loop, ansa axillaris.

Conclusion: The second pectoral nerve contributing to the pectoral loop would be better described as the caudal pectoral nerve rather than the cranial pectoral nerve. Besides the evolutionary perspectives, understanding the findings of this study would be helpful for teaching veterinary anatomy.