DISSECTION OF THE BRAIN 251 



rated from each other by a median vertical septum. 

 Posteriorly they open into the cavity of the thalam- 

 encephalon, 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 slender pineal body, behind with the 

 cavity beneath the optic lobes, and below with 

 the infundibulum. 



c. The pituitary body is a sac with very thin walls, 

 standing ia this respect in marked contrast to the 

 infundibulum, which has thick walls. Its cavity 

 communicates in front with that of the infundi- 

 bulum, and behind with that of the tubular stalk 

 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. 



e. The fourth ventricle is the cavity in the medulla, 



immediately behind the cerebellum : its sides and 

 floor are thick, its roof very thin. 



E. The Spinal Cord and Nerves. 



Slice away the roof of the neural canal horizontally, so as 

 to expose the spinal cord from the dorsal surface. 



Press away the cord gently from the side of the canal, to 

 expose the roots of the spinal nerves. 



