250 THE DOG-FISH 



e. The pituitary body is a thin-walled sac immediately 



behind the infundibulum, and attached to it. It 

 is produced laterally into two thin- walled pouches, 

 the sacci vasculosi ; and it has along its ventral 

 surface a median tubular body of unknown func- 

 tion, the anterior end of which is connected with 

 the infundibulum, and the posterior end, in the 

 natural condition of the parts, attached to the floor 

 of the skull. 



f. The roots of the third nerves arise from the ventral 



surface of the brain close to the middle line, and 

 opposite the junction of the infundibulum with the 

 pituitary body. 



Lift up the pituitary body and infundibulum so as to 

 expose the third nerves fully. 



g. The medulla is narrow in front : it widens consider- 



ably opposite the roots of the fifth and seventh 

 nerves ; and then narrows again as it passes back 

 into the spinal cord. 



h, The roots of the sixth nerves arise from the ventral 

 surface of the medulla, near the middle line, and 

 a short distance behind the level of the roots of 

 the fifth and seventh nerves. 



3. The cavities of the brain. 



Bisect the brain by a median vertical incision along its 

 whole length. Slice away the inner surface of one half of the 

 prosencephalon until the cavity within it is fully exposed. 



The brain, as already noticed, is tubular, and its 

 cavity is continued into all the principal lobes or 

 processes, which may be regarded as formed by fold- 

 ings of its wall, or rather by unequal growth at 

 various parts. The roof of the brain varies greatly 

 in thickness at different parts, but. the floor is nearly 

 uniform throughout (cf. fig. 37, p. 218.). 

 a. The lateral ventricles are a pair of large laterally 

 compressed cavities in the prosencephalon, sepa- 



