2 54 VERTEBRATE ZOOLOGY 



The dorsal wall of the third ventricle is a thin membrane which 

 is joined with the vascular pia mater and forms with it the ante- 

 rior choroid plexus ; it is situated in the bend of the fornix, and 

 may be torn away when the section is made. In the hinder part 

 of the dorsal wall, and just beneath the splenium, is a small conical 

 thickening, the pineal body, or epiphysis. 



Back of these structures is the midbrain. The extension of 

 the third ventricle backward into the midbrain is called the aque- 

 ductus Sylvii ; it will be seen to be a straight, longitudinal canal. 

 In its dorsal wall are four thickenings, called the corpora quadri- 

 gemina, which extend back to the cerebellum ; its ventral wall is 

 much thicker than the dorsal, and forms the cerebral peduncles, 

 which extend back to the pons. 



The fourth and fifth divisions of the brain are behind the mid- 

 brain. Their cavity is the fourth ventricle, which extends back 

 from the aqueductus Sylvii and is continuous posteriorly with the 

 central canal of the spinal cord. This ventricle is narrow at its 

 two ends and wide in the middle. The dorsal wall, which is just 

 beneath the cerebellum, will be seen to be very thin, and is often 

 torn away by the knife in making the section. The hinder portion 

 of it is joined with the vascular pia mater and forms the posterior 

 choroid plexus. 



The cerebellum is an outgrowth of the dorsal wall of the fourth 

 division of the brain. The peculiar branching of its numerous 

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

 section, has received the name of arbor vitae. Note the great 

 thickness of the pons which forms the ventral wall of this division 

 of the brain. 



Exercise 41. Draw the median surface of the divided brain and 

 carefully label all the parts which appear. 



The Lateral Ventricles. The two hemispheres contain the first 

 and second, or lateral, ventricles and are bound together, as we 

 have seen, by the corpus callosum and the fornix. 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, and 

 remove the hemisphere, being careful not to cut into the ventricle, 



