384 DISSECTION OF THE EABBIT. 



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 

 awa/y 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 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 mammiUare. 



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 sur- 

 face, lying between the cerebellum and the optic lobe. 

 Find the nerve on 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 

 inner and smaller one is motor, the larger outer one 

 sensory in function. 



6. The abducent nerve is very slender, and arises from the 



ventral surface of the medulla, close to the middle 

 line, and immediately behind the pons Varolii. 



