THE RABBIT 3^ 



The third ventricle is a very narrow, slitlike space which lies 

 behind the lamina terminalis ; if the section of the brain is not 

 exactly median, it may not come into view. Its lateral walls are 

 formed on each side by the optic thalamus ; ventrally the ventricle 

 is continued into the tuber cinereum, the infundibulum, and the 

 pituitary body. In the dorsal portion of the third ventricle, and 

 lying just behind the fornix, is a large body, the middle commis- 

 sure ; it connects the two thalami. The dorsal wall of this ventri- 

 cle is a thin, vascular, dark-colored membrane which forms the 

 anterior choroid plexus and may have been torn away when the 

 section was made ; rising from the hinder part of it is the small, 

 conical-stalked pineal body, or epiphysis. 



Posterior to these structures is the mesencephalon, or midbrain. 

 The extension of the third ventricle posteriorly in the midbrain is 

 called the aqueductus Sylvii; this serves to connect the third 

 and fourth ventricles. Its thickened dorsal wall is formed by the 

 corpora quadrigemina, and the still more thickened ventral wall 

 by the cerebral peduncles. 



The fourth ventricle lies in the medulla oblongata and is con- 

 tinuous posteriorly with the central canal of the spinal cord. Its 

 dorsal wall, which lies beneath the cerebellum, is a thin, vascular, 

 dark-colored membrane which forms the posterior choroid plexus ; 

 it may have been removed when the section was made. 



The cerebellum is an outgrowth of the dorsal wall of the meten- 

 cephalon, the fourth division of the brain. The peculiar branch- 

 ing of the folds, especially of the white matter as it appears in a 

 longitudinal section, has received the name of the arbor vitae. 

 Note the great thickness of the pons, which forms the ventral wall 

 of this division of the brain. 



Exercise 33. Draw the median surface of the bisected brain and 

 carefully label all the parts observed. 



The Lateral Ventricles. The two hemispheres contain the first 

 and second, or lateral, ventricles of the brain. Introduce the blade 

 of a small scalpel between the upper surface of the corpus callosum 

 and the hemisphere of the left half of the brain, being careful not 

 to cut into the ventricle, the roof of which is formed by the corpus 



