82 



THE NERVOUS SYSTEM OF THE FROG. 



V. 



VI. 



touching each other in the median plane, and form- 

 ing the widest part of the brain : the pia mater 

 covering them is very strongly pigmented. 



The cerebellum is a narrow transverse band im- 

 mediately behind the optic lobes. 



The medulla oblongata is the part of the brain 

 behind the cerebellum : it is widest in front and 

 gradually tapers towards its posterior end, wh^re 

 it is continuous with the spinal cord. It is 

 covered by a triangular and very vascular mem- 

 brane, beneath which lies the fourth ventricle. 



Fig. 16. A horizontal section through the brain of the frog, to show 

 the internal cavities. (From Ecker.) 



Aq, ventricles of the optic lobes, and Sylvian aqueduct : Dv% third 

 ventricle : MF, foramen of Monro : Sv, lateral ventricle : Vv, fourth 

 ventricle. 



b. The cavities of the brain. 

 Slice of the upper surface of the brain horizontally so as to 

 expose the several cavities or ventricles, without removing it from 



