xiii.] THE FROG. 187 



constriction from the olfactory lobes. The outer wall of 

 the ventricle, though relatively thick, presents nothing which 

 can be called a distinct corpus striatum. The inner wall 

 forms one or two convex projections into the ventricle. 



In the bases of the olfactory lobes the forward continua- 

 tion of the ventricular cavity is very narrow and the lobes 

 become nerve-like cords, which leave the skull and spread 

 out on the posterior faces of the olfactory sacs. 



The inner faces of the hemispheres are quite free and 

 separated by a cleft, the great fissure, but the inner faces of 

 the commencements of the olfactory lobes are closely united 

 together, giving rise to a kind of corpus callosum. 



There are ten pairs of cranial nerves ordinarily so called, 

 though it is to be recollected that the first and second pairs 

 are proved, by their development, to be lobes of the brain. 



1. O If actor il. 



The olfactory lobes are what answer to the so-called 

 olfactory nerves of the higher Vertebrata. They are 

 distributed exclusively to the olfactory sacs. 



2. Optici. 



These diverge from the base of the brain in front of 

 the infundibulum. They are originally outgrowths 

 of the thalamencephalon which secondarily become 

 connected with the optic lobes. 



Of the remaining cranial nerves five pairs leave the skull 

 in front of the auditory capsules, while one pair enters those 

 capsules and two pairs pass out behind the capsules. 



The Pr&auditory nerves are the following. 



3. Mot ores oculorum 



arise from the front part of the floor of the mid-brain 

 and are distributed to all the muscles of the eye 



