Vertebral muscles of the back and tail of the albino rat (Rattus norvegicus albinus).

The dorsal and ventral vertebral muscles of the back and the tail of the albino Norway rat are described and discussed. These muscles were analyzed because they are undoubtably used during the sexual posturing, lordosis, of the female rat, as well as participating in a variety of other behaviors. The muscles are described by region (thoracic-lumbar or sacrocaudal), and the classifications of Vallois are followed where possible. Of the epaxial (dorsal) muscles, the three longitudinal systems of muscles, the transversospinalis, the longissimus, and the iliocostalis systems, can be identified in the albino rat. Muscles of the transversospinalis system are fused in the lumbar region, distinct and specialized in the thoracic region, and form the tail muscle extensor caudae medialis caudally. The iliocostalis system of the lumbar region is fused with one component of the lumbar longissimus system to form lateral longissimus. Anteriorly, iliocostalis thoracis and cervicis represent the iliocostalis system. The lumbar longissimus system is represented by the longissimus component of lateral longissimus, medial longissimus, and a short-fiber component. Longissimus dorsi is the anterior continuation of the longissimus portion of the lateral longissimus. The short-fiber component also continues into the thoracic region, where it becomes difficult to separate out from longissimus dorsi. Medial longissimus represents the excursion into the lumbar region of the long, tendinous, tailbase-tail muscle, longissimus caudae; the caudal portion of this muscle is extensor caudae lateralis. The remaining dorsal muscle described is the tail muscle, abductor caudae dorsalis. The hyposomal (ventral) muscles described are quadratus lumborum and the intertransversarii, present in the lumbar region; the muslces iliococcygeus, pubococcygeus and coccygeus which arise from the medial face of the pelvis and insert onto the proximal tail; the long, tendinous, tail muscles, flexor caudae brevis and longus; and the ventral and lateral set of segmental tail muscles. The innervation of the lumbar transversospinalis muscles by the medial branches and of lateral longissimus by the lateral branches of the dorsal rami of the spinal nerves was traced and confirmed by electrical stimulation of the nerve branches. The innervation of medial longissimus is also described. Additionally, movements of the vertebral column produced by direct unilateral or bilateral muscle stimulation were observed for a number of the muscles.