84 DISSECTION OF THE BRAN, 
gets into the interior through the great transverse. 
fissure of the brain between the cerebrum and cere- 
bellum. At the anterior end of the ventricle it con- 
tinues on through the foramen of Monro on each 
side into the lateral ventricles, forming the chorozd 
plexuses. Each choroid plexus is a thin vascular fold 
of membrane which passes backward into the descend- 
ing horn of the lateral ventricle upon the hippocampus 
major. If the corpus callosum is removed with suffi- 
cient care these relations of the pia mater can easily 
be demonstrated upon the dog’s brain. 
1, THE 3D VENTRICLE Is seen as a narrow slit begin- 
ning just back of the anterior pillars of the fornix, 
and extending posteriorly as far as the corpora 
quadrigemina. While narrow from side to side, it is 
quite deep. 
2. The Optic Thalami (¢halamencephalon) are the 
two oval masses forming the sides of the 3d ventricle. 
a. THE PINEAL GLAND is. connected by a stalk to 
the upper and posterior end of the 3d ventricle. It 
was originally a diverticulum from this ventricle. 
b. THe CoMMISSURES OF THE 3D VENTRICLE. 
The Wzddle Commissure is very large, but delicate 
and easily broken. It passes across the middle of the 
ventricle between the optic thalami. 
The Posterzor Commussure is at the extreme pos- 
terior end of the 3d ventricle, lying beneath the stalk 
of the pineal gland; It is anarrow band of white fibres. 
The Axterzor Commzssure lies at the extreme ante- 
rior end of the 3d ventricle, just where the anterior 
pillars begin to diverge from each other. By cutting 
the body of the fornix open vertically this commis- 
sure can be brought into full view. It isa narrow 
band of white fibres. 
