72 THE NERVOUS SYSTEM OF THE FROG 



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. 



v. The cerebellum is a narrow transverse band im- 

 mediately behind the optic lobes. 



vi. The medulla oblongata is the part of thfe brain 

 behind the cerebellum : it is widest in front and 

 gradually tapers towards its posterior end, where 

 it is continuous with the spinal cord. It is 

 covered by a triangular and very vascular mem- 

 brane, beneath which lies the fourth ventricle. 



Vv 



FIG. 15. A horizontal section through the brain of the frog, to 

 show the internal cavities. (From Ecker. ) 



Aq, ventricles of the optic lobes ; Dv, third ventricle ; MF, foramen 

 of Monro ; Sv, lateral ventricle ; Vv, fourth ventricle. 



b. The cavities of the brain. 



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

 expose the several cavities or ventricles, without removing it from 



