The Anterior Cardinal Sinus. (Fig. 4.)

In Typhlonarke (Fig. 4, C) the anterior cardinal sinus (a.c.s.) stretches from the back of the auditory capsule to the end of the ductus Cuvieri (d.c.). Anteriorly it receives the hyoidean sinus (h.s.), which connects it to the jugular sinus (j.s.), the post-orbital sinus (p.o.s.) and the posterior cerebral vein (p.c.v.). Along its length a vein is received from each gill-bar, which, like the hyoidean vein, extend from the anterior cardinal to the jugular. All these receive veins from the electric organ (e.v., Fig. 5). Cutaneous veins draining the gill pouches also enter the anterior cardinal. The post-orbital sinus is a narrow vessel running lateral to the auditory capsule, ventral to an overhanging ledge of cartilage, and connecting the anterior cardinal to the orbital sinus. The posterior cerebral vein passes through the cranium with the vagus and the vein on the right is larger than the one on the left. Each vein divides into two, the anterior branch on each side uniting with the one from the other side in front of the cerebellum, from which point a vein runs forward along each side of the brain and receives branches from its surface. The posterior branches of the cerebral veins join behind the cerebellum and pass back along the spinal cord as the myelonal vein.

A vein which drains the jaw muscles enters the post-orbital sinus. The orbital sinus receives laterally a vein which branches about the spiracle and another supplying the eye. The interorbital sinus (i.o.s.) which is very well formed, leaves the orbital sinus just posterior to the articulation of the rostral cartilages with the brain case. It runs for about half an inch on the antero-lateral inside wall of the brain case before passing ventrally through the cartilage to the under surface, where it unites with the interorbital sinus of the opposite side. Ventral to the brain ease branches are received from the olfactory organ. This condition is very different from that described by O'Donaghue for Scyllium canicula in which “the interorbital vein is a small but well marked vein running from one orbit to the other in a canal in the basicranial cartilage, and it enters the orbit toward its posterior end just in front of and slightly below the large foramen in the cranium through which the sixth and main branches of the fifth and seventh nerves leave the cranium.” In Typhlonarke it is much further forward, and for a part of its length runs on the inside of the cranium.

In the Stingray (Fig. 4, A) the anterior cardinal sinus receives anteriorly the post-orbital sinus, the posterior cerebral vein and from the gill-bars, the hyoidean and branchial veins as in Typhlonarke. Medianly four cutaneous veins (c.v.) enter the anterior cardinal sinus. Dorsally they unite into a longitudinal vessel which enters the ductus Cuvieri. The post-orbital sinus also receives a cutaneous vein quite close to the point at which a pre-spiracular vein also enters the sinus.