i36 ELEMENTARY BIOLOGY. [CHAP. 



with an opening on its inner face. These openings lead into 

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

 quartum 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 repre- 

 sents 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 hemi- 

 spheres. 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 pituitary 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 hemisphere 

 the lateral ventricle. 



The hemispheres are elongated bodies, broader behind 

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



