248 THE DOG-FISH 



4. The optic lobes are a pair of closely apposed ovoidal 

 protuberances, placed in the narrowest part of the 

 cranial cavity opposite the middle of the orbits. Their 

 posterior halves are overlapped and concealed by the 

 cerebellum. 



6. The cerebellum is a long flattened oval body arising from 

 the dorsal surface of the brain immediately behind 

 the optic lobes, and extending forwards over the optic 

 lobes and backwards for an equal distance over the 

 medulla. Its surface is slightly grooved. 



6. The medulla oblongata is the part of the brain behind the 

 cerebellum. Its roof is very thin, and liable to be torn 

 away so as to expose its cavity, the fourth ventricle. 

 Its sides are thickened, especially in front, where 

 they give origin to the roots of the fifth, seventh, 

 and eighth cerebral nerves : immediately above these 

 roots, and below the posterior projection of the cere- 

 bellum, the medulla is produced into a pair of large 

 lateral wing-Hke processes, the restiform bodies, 

 which are united with each other beneath the over- 

 hanging cerebellum. 



B. The Orbit. 



This should be examined before the dissection of the nerves 

 is proceeded with. If the eyelids have been removed as 

 directed above, no further dissection is necessary. Wash out 

 the blood-clots from the orbital sinus with a stream of water, 

 avoiding injury to the nerves. 



1. The eyeball is almost hemispherical in shape, its outer 



or corneal surface being very flat. 



2. The eye-muscles, by which the movements of the eye 



are effected, are six narrow muscular bands which 



arise from the skuU-wall and are inserted into the 



eyeball : they are arranged in two groups. 



a. The recti muscles are four in number. They arise 



close together, near the hinder end of the orbit, 



and diverge outwards to be inserted into the 



eyeball. 



