8o LABORATORY DIRECTIONS 



located at the base of the cerebral hemispheres. This is the 

 largest commissure of the brain. The anterior and posterior 

 ends are slightly swollen. A band of fibers, the fornix, extends 

 ventrally from about the middle of the corpus callosum. The 

 anterior commissure is a small cord of fibers cranial to the ventral 

 end of the fornix. The middle commissure is a large bundle of 

 fibers posterior to the fornix. It is located in the middle of 

 the third ventricle and is part of the thalamus, to be mentioned 

 later. The posterior commissure is a small group of fibers 

 posterior to the middle commissure. 



The ventricles of the brain of the cat occupy positions similar 

 to those of other vertebrates. The lateral ventricles are small 

 cavities in the cerebral hemispheres. They may be found by 

 making a longitudinal slit through the corpus callosum and 

 lifting up the cerebral lobes. Each lateral ventricle connects 

 by a foramen of Monro to the third ventricle, which surrounds 

 the middle commissure and extends into the pituitary gland. 

 The iter is a narrow passage under the cerebellum, connect- 

 ing the third and fourth ventricles. The fourth ventricle is the 

 cavity of the medulla and is continuous with the canal of the 

 spinal cord. 



Anterior to the cerebellum and dorsal to the iter are two 

 rounded swellings in each half of the brain. These are the 

 corpora quadrigemina, corresponding to the optic lobes of lower 

 vertebrates. The cerebral peduncles form the floor of the iter. 



The branched white tracts in the cerebellum constitute 

 the arbor vitae. 



Remove the dorsal wall of the cerebral hemisphere of one 

 half of the brain. In doing so observe that the outside of the 

 brain is gray and the inside white. The gray layer is the cortex. 

 Identify again the corpora quadrigemina. The prominent 

 elevation anterior to these is the thalamus, a large ganglion. 

 Anterior to the thalamus is another ganglion, the corpus striatum, 

 which forms part of the floor of the lateral ventricle. 



Examine the series of vertebrate brain models and identify 

 the principal structures on each. Observe the marked differ- 

 ences in some of the parts. 



