274 THE DOG-FISH. 



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 projec- 

 tion of the cerebellum, the medulla is produced into 

 a pair of large lateral wing-like processes, the resti- 

 form bodies, which are united with each other beneath 

 the overhanging 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 skull-wall and are inserted into the 

 eyeball : they are arranged in two groups. 



