THE SKATE. 73 



of the prosencephalon, and passing forwards and slightly 

 outwards to the olfactory foramina (§ 32). Their further 

 course will be seen at a later stage (§ 206). 



180. By lifting up or removing the cerebellum, the 

 whole extent of the fourth ventricle or myelocoele 

 (Figs. 22 and 24, v. 4) is seen : viewed from above it has a 

 triangular shape, the base being forwards, and the apex 

 (calamus scriptorius) backwards, as already seen. 



Fig. 23. — Raja batis. The brain from beneath (half nat. size). 



Only the proximal portions of the olfactory lobes are shown. 



>». (J, medulla oblongata : //, pituitary body : s.v, saccus vascolosus : 

 l.i, lobi inferiores : i.c, internal carotid artery : o.c, optic chiasma : c.h, 

 prosencephalon : /— JT, cerebral nerves. 



181. By carefully cutting away the optic lobes, the fourth 

 ventricle is seen to be continuous with a cavity in the 

 mesencephalon, the aqueduct of Sylvius or mesocoele 

 (Fig. 24, aq. s), which sends lateral prolongations into the 

 optic lobes. Anteriorly the mesoccele is seen to be con- 

 tinuous with the thalamocoele (». 3). 



182. By cutting into the prosencephalon, it is seen to be 

 solid, lateral ventricles (prosocceles) being absent. 



183. From the under surface of the thalamencephalon 

 arise the two large optic (second cerebral) nerves (Figs. 22. 



