178 ELEMENTARY BIOLOGY. [siII. 



a short passage, which communicates with the iter a tertio ad- 

 qiiartum ventriculum, as the canal which leads, through the 

 mesencephalon, from the fourth to the third ventricle is 

 termed. The floor of this canal is formed by the thick prin- 

 cipal mass of the cerebro-spinal axis. It exhibits a median 

 longitudinal depression or raphe, and in this region represents 

 the crura cerebri. 



In front of the mid-brain comes the hinder division of the 

 fore-brain, or thalamencephalon, which is very distinct in the 

 Frog and contains a median cavity, the third ventricle. On 

 each side, the cavity of the third ventricle is bounded by a 

 thick mass of nervous matter into which the crura cerebri 

 pass. These are the optic thalami. Dorsally, the walls of 

 the third ventricle are very thin and easily torn through, 

 except behind, where there is a thick transverse band of 

 nervous substance, the posterior commissure. 



From the fore part of the roof of the third ventricle, a 

 delicate process proceeds to the pineal gland — an ovate body 

 lodged between the posterior parts of the cerebral hemispheres. 

 The front part of the floor of the ventricle, on the other hand, 

 is produced into a bilobed process directed backwards, which 

 is the infundibulum. This is connected below with the pitui- 

 tary body. In front of this is seen the commissure of the 

 optic nerves. 



Anteriorly, the third ventricle is bounded by the thick 

 lamina terminalis which contains the anterior commissure. 

 On each side, between this and the peduncle of the pineal 

 gland, is a small aperture, the foramen of Munro, which leads 

 into a cavity in the interior of the cerebral hemisiDhere^the 

 lateral ventricU. 



The hemispheres are elongated bodies, broader behind 

 than in front, where they are marked off only by a slight 

 constriction from the olfactory lobes. The outer wall of the 

 ventricle, though relatively thick, presents nothing which can 



