The Caval Sphincter in Phoca vitulina L.

Abstract

EXAMINATION of dissections and casts of the venous system of twenty-five specimens of the common seal has confirmed, in the main, the findings of previous workers¹. In addition, the casts revealed the existence of a large vein lying in the spinal canal external to the dura mater. In the lumbar and thoracic regions this extradural vein is single and lies dorsal to the cord; it receives three major tributaries from each renal plexus, and also communicates with almost every segmental vein of the trunk and with the vena azygos. In the cervical region it divides into two smaller veins which lie one on each side of the cord, with numerous anastomosing channels passing between them. These two veins unite deep to the posterior arch of the atlas, and the sinus thus formed communicates with the intracranial sinuses by two veins, each of which passes through the corresponding condylar canal.