Evolutionary comparative analysis of the extrinsic thoracic limb muscles in three procyonids (Procyon cancrivorus Cuvier, 1798, Nasua nasua Linnaeus, 1766, and Potos flavus Schreber, 1774) based on their attachments and innervation

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.

Origin and Distribution of the Brachial Plexus in Two Procyonids (Procyon cancrivorus and Nasua nasua, Carnivora)

Procyon cancrivorus and Nasua nasua are two procyonids with different evolutionary adaptations to use their thoracic limbs. Therefore, this study aimed to characterize the differences in the brachial plexus between both species. Five P. cancrivorus and five N. nasua cadavers were used to perform this investigation with the permission of the bioethics committee and environmental license. Gross dissections were performed on the cervical, pectoral, and thoracic limb regions to find the origin and distribution of the brachial plexus. The brachial plexus of both species originated in a variant manner from C5-T1, C5-T2, C6-T1, or C6-T2. All brachial plexus nerves were observed and, interestingly, the musculocutaneous sent a communicating branch to the median nerve medially to the axillary artery, forming an ansa axillaris in both species. An ansa pectoralis was also observed medially to the axillary artery. Additionally, in P. cancrivorus, the musculocutaneous nerve innervates the pronator teres and flexor carpi radialis muscles and communicates with the median nerve at the elbow level to continue as a common trunk at the antebrachium. The brachial plexus has differences between both procyonids, although in both species, it could conserve a primitive arrangement present within the infraorder Arctoidea.