284 THE DOG-FISH. 



natural condition of the parts, attached to the floor 

 of the skull. 



It is important to remember that the pituitary 

 body, as here defined, is not developed from the 

 brain. 



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 oblongata is narrow in front : it 



widens 'considerably 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. 51, p. 247). 



a. The lateral ventricles are a pair of large laterally 

 compressed cavities in the prosencephalon, sepa- 

 rated from each other by a median vertical 

 septum. Posteriorly they open into the cavity 

 of the thalamencephalon, and on their outer 

 sides they are prolonged into the olfactory lobes. 



