Updating the forelimb anatomy of the domestic cat (Felis catus, Felidae) based on evolutionary inferences of its muscles and nerves I: Shoulder and brachium.

The domestic cat (Felis catus) is a species of the order Carnivora, suborder Feliformia, and family Felidae. It is used as a representative model of mammals in comparative anatomy of vertebrates, whereas in veterinary anatomy, only differences from the domestic dog are described. Because of this, several recent studies on thoracic limb muscles in wild felids have preferred the use of former references concerning the anatomy of the cat as a guide to describe their findings. Furthermore, these studies established the homologies of the thoracic limb muscles among felids based on topology but did not consider other evolutionary characteristics, such as innervation and embryological development. Therefore, the present study aimed to describe in detail the thoracic limb muscles and the brachial plexus of F. catus to reconstruct muscle maps, report anatomical variants, establish homologies, and infer muscle derivations in felids. In this chapter, we report the attachments, innervation, and arterial supply of the intrinsic shoulder and brachium muscles and the origin and distribution of the brachial plexus of F. catus in these regions. Nineteen thoracic limbs from 10 carcasses of F. catus were used. Differences from other studies in F. catus and other felids were observed when considering the topology, innervation, and most recent available literature on the embryonic muscle splitting patterns in amniotes. Among the main findings, we observed the articularis humeri muscle in only one limb (1/19), exhibiting the arrangement of an atavistic muscle, similar to the coracobrachialis longus muscle observed in six limbs (6/19) and the caput breve of biceps brachii muscle observed in two limbs (2/19). The triceps brachii muscle only has four heads, and the anconeus medialis muscle (named by other authors as the short portion of the caput mediale, caput mediale accessorium, or m. epitrochleoanconeus) is not part of the triceps brachii muscle since its attachments are independent, the ulnar nerve innervates it, and the most recent embryological derivation evidence in mammals supports it. Two tricipital bursae were observed between the tendons of the triceps brachii heads. The muscle maps on the thoracic limb bones were reconstructed and complemented with the anatomical relationships of the ligaments, joint capsules, synovial bursae, and nerves. Most studied brachial plexuses originated from C6-T1 (13/16), whereas the origin from C5 was observed bilaterally in two limbs and from T2 in one limb. Adding the results of other studies (56), the contributions from C5 or T2 were 4.17% (3/72) and 1.39% (1/72), respectively. The distribution of the brachial plexus nerves of F. catus was similar to that reported in other studies in felids, although most of those studies did not report innervation to the anconeus medialis muscle. The nerve distribution and muscle topology allowed us to establish muscle homologies with other felids and infer evolutionary derivations of the shoulder and brachium muscles in the family Felidae. In conclusion, the present study updated the anatomy of the shoulder and brachium in F. catus and should be considered in evolutionary studies in Carnivora and diagnoses and surgical approaches in veterinary medicine of felids.