DISSECTION OF THE BRAIN. 79 



2. The Ventral Surface of the Brain. 



a. The Cekehral Hemispheres meet in front, but 

 diverge posteriorly. The vertical fissure separating the 

 hemispheres (on the dorsal side) is continued around 

 for some distance on the ventral surface. Note the 

 large olfactory lobes lying along the ventral surface. 



b. The Optic Chiasma. Lying at the end of the fis- 

 sure is a transverse band of nerve-fibres, the chiasma. 

 The optic nerves arise from it anteriorly, and poste- 

 riorly it passes backward and outward, forming the 

 optic tracts, which finally end in the corpora quadri- 

 gemina. By carefully lifting up the side of the cere- 

 bral hemisphere each tract may be followed as it passes 

 over the optic thalamus, in which some of the fibres 

 end, until it reaches the corpora quadrigemina. 



c. The Lamina Cinerea is exposed by turning 

 back the optic chiasma. As will be seen later, it forms 

 the anterior wall of the third ventricle. If gently torn, 

 the third ventricle will be exposed. 



d. The Infundibulum lies immediately behind the 

 chiasma. It is a median prolongation of the ventral 

 surface of the brain : at its apex is \\\^ pituitary body. 

 The infundibulum is hollow within, the cavity being 

 a prolongation of the third ventricle. 



e. The Corpora Albicantia are two small white 

 eminences posterior to the infundibulum. 



f. The Crura Cerebri are the two eminences 

 lying outside of and posterior to the corpora albicantia. 

 They pass forward and upward into the cerebral 

 hemispheres, diverging from each other ; they form 

 the connection between the cerebral hemispheres and 

 the medulla. (To expose them clearly the pia mater 

 must be cleaned off, taking care not to injure the 

 cranial nerves.) 



