DISSECTION OF THE. BRAIN, 79 
2. The Ventral Surface of the Brain. 
a. THE CEREBRAL 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 Curasma. Lying at the end of the fis- 
Sure 1s a transverse band of nerve-fibres, the chiasma. 
The oftzc 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 1s 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 the pztuztary 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. ) 
