258 THE DOG-FISH 



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. 49, p. 224.). 



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. 

 The posterior wall of the prosencephalon, 

 just in front of and above the thalamencephalon, 

 is produced on each side into a very thin-walled 

 fold, which projects forwards into the lateral 

 ventricle nearly to its anterior end. Between 

 the two layers of the fold are numerous blood- 

 vessels forming the choroid plexus. 



b. The third ventricle is the cavity within the thalam- 



encephalon : it communicates in front with the 

 lateral ventricles, above with the very narrow 

 cavity of the stalk of the pineal body, behind 

 with the cavity beneath the optic lobes, and be- 

 low with the infundibulum. 



D, The pituitary body is a sac with very thin walls, 

 standing in this respect in marked contrast to 

 the infundibulum, which has thick walls. Its 

 cavity communicates in front with that of the in- 

 fundibulum, and behind with that of the tubular 

 body which lies on its ventral surface. 



d. The optic lobes, cerebellum, and restiform bodies 



are all hollow outgrowths from the dorsal surface 

 of the brain. Their walls are of moderate thick- 

 ness, excepting the dorsal ends of the restiform 

 bodies, which are very thin. 



