SPINAL CORD AND NERVES. 285 



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 a 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 

 below with the infundibulum. 



c. The saccus vasculosis is a sac with very thin walls, 



standing in this respect in marked contrast to 

 the infundibulum, which has thick walls. Its 

 cavity does not communicate with the pituitary 

 body which lies on its ventral surface. The 

 pituitary body has only a secondary connection 

 with the brain. 



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, 



beneath and 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 dorsal surface of the spinal cord. 



Gently press the cord away from the side of the canal, to 

 expose the roots of the spinal nerves. 



1. The spinal cord lies in the neural canal of the vertebral 

 column. It is flattened dorso-ventrally ; marked by 



X 



