Due to the absence of literature on the brachial plexus in the Spix's yellow-toothed cavy, this study was conducted to examine the origin and neural distribution of this rodent's brachial plexus. Ten adult animals obtained from previous experiments were thawed, fixed in 10% formalin solution for 72 h and then dissected. Cotton blocks soaked in 2% hydrogen peroxide were placed on the ventral roots of the plexus. Photographs of the most representative specimens were taken and schematic sketches were prepared to illustrate the results. The Spix's yellow-toothed cavy's brachial plexus primarily originated from the ventral roots of the sixth (C 6 ), seventh (C 7 ) and eighth (C 8 ) cervical nerves and from the first two thoracic nerves (T 1 , T 2 ), and less frequently from C 6 -T 1 or C 6 -T 3 . The peripheral nerve trunks of the Spix's yellow-toothed cavy's brachial plexus are the following: suprascapularis (C 6 -C 7 ), axillaris (C 6 -C 7 ), subscapulares (C 6 -C 7 ), cranial pectoral (C 6 -C 7 ), musculocutaneous (C 7 -C 8 ), radialis (C 7 -T 2 ), medianus (C 7 -T 2 ), ulnaris (C 7 -T 2 ), pectorales caudales (C 7 -C 8 ), thoracicus longus (C 7 -C 8 ), thoracicus lateralis (C 7 -T 2 ) and thoracodorsalis (C 7 -T 2 ).Keywords: anatomy; brachial plexus; nervous system; rodent; Galea spixii List of abbreviations C 6 = ventral root of C 6 ; C 7 = ventral root of C 7 ; C 8 = ventral root of C 8 ; T 1 = ventral root of T 1 ; T 2 = ventral root of T 2 ; T 3 = ventral root of T 3

The Mongolian squirrel is a murine rodent widely used as an experimental model and kept as a pet. With respect to the significance of the brachial plexus, we aimed to describe the origin and the nerves that constitute it, providing new information about the neuroanatomy of this species. Twenty adult animals were used, which were fixed in a 10% formaldehyde solution at the Applied Animal Morphophysiology Laboratory of the Brazilian Federal University of the Semi-Arid Region (UFERSA). The animals were medially incised with an opening of the thoracic cavity, and then bilateral dissection was carried out. Once the nerves were identified, cotton wool soaked with a 2% hydrogen peroxide solution was added for bleaching and better visualisation of structures. The gerbil brachial plexus derived from ventral roots of the fourth, fifth, sixth, seventh and eighth cervical nerves (C₄–C₈) and the first thoracic root (T₁). The suprascapular nerve was formed from the nerve roots C₄ and C₅; the subscapular and cranial pectoral nerves from C₅ and C₆; the musculocutaneous and axillary nerves from C₅, C₆ and C₇; and the caudal pectoral, radial, median, ulnar, thoracodorsal, lateral thoracic and long thoracic nerves from C₇, C₈ and T₁. This resembles what is described in other rodents, such as the rat and capybara. The gerbil has a pre-fixed plexus conformation.