THE ELASMOBRANCII FISHES 



207 



considerably from the condition seen in Heptanchus. It is formed by an 

 anterior vessel on the olfactory bulb which receives a branch from the olfac- 

 tory lobe, a median branch from the ventral olfactory and diencephalic areas, 

 and a posterior vein which in the middorsal line joins a similar vein from the 

 opposite side as the mesencephalic vein. In other words, the first branch of 



fo. VII 



po p. 



Fig. 192a. Veins dorsal to roof of 

 cranium, Squalus sucMii. 



Fig. 192b. Veins in roof of buccal 

 cavity, Squalus sucldii. 



bp.v., buccopharyngeal vein ; dl.v., dorsolateral rostral vein ; dm.v., dorsomedian rostral 

 vein ; fn., basal fenestra ; fo. VII, exit of superficial ophthalmic nerve ; Us., attachment, 

 superior levator labialis muscle ; n.a., nasal aperture ; n.c, nasal capsule ; nm.v., nasomaxil- 

 lary vein; n.v., nasal vein; on.v., orbitonasal (facial) vein; p.f., parietal fossa; po.p., post- 

 orbital process; s., sinus; so.v., supraorbital vein; sr.v., subrostral vein. 



the anterior cerebral drains the olfactory lobe and tract, the second the whole 

 of the ventral area back to the optic chiasma, and the third the whole of the 

 remaining dorsal region anterior to the cerebellum. It is through the last- 

 named vessel that the roof of the third ventricle is drained. 



The anterior cardinal sinus (fig. 190) receives the hyoidean vein and the 

 nutrients (not shown) from all the holobranchs. These nutrients, as in Hep- 

 tanchus, are continuous with ventral nutrients. 



The posterior cerebrals (p.c.v., fig. 191) from the brain enter the anterior 

 cardinal sinus. In a type like Scyllhim (fig. 191a) they collect the blood from 

 the cerebellum and medulla and pass it posteriorly in large right and left 

 dorsal veins through the foramina with the vagus nerves. In Raja asterias 

 (fig. 191b) the posterior cerebral is usually a single vessel. Continuing pos- 



