I.] THE FROG. 27 



vascular membrane or pia 7nater. This becomes immensely 

 tliickened, above the hind-brain and on the inner faces 

 of the hemispheres, to constitute the choroid plexuses. The 

 former of these is median and overlies the greater part 

 of the fourth ventricle. The latter is paired ; in its growth 

 each half forces its way into the lateral ventricle, pushing the 

 thin inner wall of the same in front of it ; while therefore 

 it lies, in a sense, within that cavity, it is in reality 

 outside it. 



There are ten pairs of cranial nerves ordinarily so-called, 

 the first two of which are proved, by their development, to 

 be lobes of the brain. 



1. O (factor ti. 



The olfactory lobes. Their nerves 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. They are distributed 

 exclusively to the retina of the eye. 



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 same. 



The Prceauditory nerves are the following. 



3. Mot ores oculorwn 



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

 and are distributed to all the muscles of the eye 

 except the external rectus, the superior oblique and 

 the retractor bulbi. 



