390 DISSECTION OF THE RABBIT. 



h. The medulla oblongata, as seen from the ventral 

 surface, is broad in front and gradually narrows 

 behind. It is marked by a median ventral fissure, 

 bordered by two narrow bands, the ventral 

 pyramids. At the outer sides of the pyramids, 

 immediately behind the pons Varolii, are a pair 

 of oblong patches of transverse nerve-fibres, the 

 corpora trapezoidea. 



C. The Roots of the Cerebral Nerves. 



There are twelve pairs of cerebral or cranial nerves in the 

 rabbit, all of which, with the exception of the fourth pair, 

 arise from the ventral surface or from the sides of the brain. 



Identify, and clean the nerve roots on one side of the brain, 

 in the order given below, taking great care not to drag them away 

 from the brain. 



1. The olfactory nerve arises, by many roots, from the 



ventral surface of the anterior end of the olfactory 

 lobe. These roots are almost necessarily left be- 

 hind in taking out the brain. 



2. The optic nerve is a stout nerve, running forwards from 



the anterior border f 'of the optic chiasma, and then 

 curving outwards. 

 3 The oculo-motor nerve is small, and arises from the inner 

 border of the crus cerebri, close to the middle line, 

 and immediately behind the corpus mammillare. 



4, The pathetic nerve is very small. It arises from the 



dorsal surface of the valve of Vieussens, close to 

 the middle line, and just behind the optic lobes. 

 It then runs round the side of the brain to the 

 ventral surface, lying between the cerebellum and 

 the optic lobe. 

 Find the nerve en the ventral surface, and follow it round 

 the side of the brain to its origin. 



5. The trigeminal nerve is very large, and arises by two 



closely apposed roots from the side of the hinder 

 border of the pons Varolii. Of the two roots the 



