240 THE DOG-FISH 



The brain is large and very nearly fills the cranial cavity : 

 it is narrow between the orbits, but expands considerably in 

 front of them. As seen from above, the following parts are 

 visible in order from before backwards (c/. fig. 37, p. 218). 



1. The prosencephalon is the most anterior and the largest 



division of the brain. It is a smooth, somewhat 

 globular mass, lying in the expanded anterior part of 

 the cranial cavity, between the olfactory capsules and 

 the orbits. It is marked in front by a shallow median 

 groove ; and is very sharply separated behind from 

 the succeediag part of the brain. It corresponds to 

 the cerebral hemispheres of higher vertebrates. 



2. The olfactory lobes are a pair of large oval masses arising 



by stout stalks from the sides of the prosencephalon, 

 and having their distal surfaces closely applied to the 

 olfactory capsules. 

 8. The thalamencephalon is the narrow portion of the brain 

 immediately behind the prosencephalon ; its roof is 

 very thin, so that the cavity, or third ventricle, 

 can be seen through it. From this roof arises the 

 pineal body, a slender tubular process which extends 

 forwards over the prosencephalon, and ends in a 

 slightly dilated knob attached to the roof of the skull 

 some little distance in front of the brain. 



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. 



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 the optic 

 lobes and backwards for an equal distance over the 

 meduUa. 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. 



