266 THE DOG-FISH. 



cephalon, and ends in a slightly dilated knob attached 

 to the membranous roof of the skull some little 

 distance in front of the brain. 



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



protuberances lying in the narrowest part of the 

 cranial cavity opposite the middle of the orbits. 

 Their posterior halves are overlapped and concealed 

 by the cerebellum. 



5. 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 them, 

 and backwards for an equal distance over the 

 medulla oblongata. 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-like 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 tvith a stream of luater, 

 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 skull-wall and are inserted into the 

 eyeball : they are arranged in two groups. 



