THE DOGFISH 



3°5 



fontanelle. The floor of the thalamencephalon bears a 

 hollow, backwardly directed infundibulum, which differs 

 from that of the frog in being folded, the end passing 

 forwards under the first part, and in bearing at the sides 

 a pair of thick-walled lobi inferiores and behind these a 

 three-lobed, thin-walled, vascular expansion known as 

 the saccus vasculosus.. The. pituitary body is said to be 



, Sinus venosus 

 > Auricle 



Ventricle 

 Conus arteriosus 

 Venfral arterial sysfem 



Gills 



4 , 

 Dorsal arterial sysfem 



Head a. 



Fig. 261. — A diagram of the circulation of the blood in a dogfish. 

 Thick lines indicate venous blood, narrow lines arterial blood. 



a very small tube which lies below the infundibulum 

 attached to the floor of the skull. The mid-brain, which 

 succeeds the thalamencephalon, bears above two optic lobes 

 which stand closer than those of the frog. The cerebellum 

 behind them is much larger than that of the frog and oval 

 in outline, with the long axis fore and aft, and overhangs 

 the optic lobes in front and the thin-roofed fourth ventricle 

 in the medulla oblongata behind it. The medulla is pro 

 duced forward into a pair of wings, the restiform bodies 

 which lie at the sides of the cerebellum. 



