84 DISSECTION OF THE BRAIN. 



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 ot Monro on eacn 

 side into the lateral ventricles, forming the choroid 

 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. 



i. 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 {thala7nencephalon) 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 30 Ventricle. 

 The Middle Commissure is very large, but delicate 



and easily broken. It passes across the middle of the 

 ventricle between the optic thalami. 



The Posterior Coimnisszire is at the extreme pos- 

 terior end of the 3d ventricle, lying beneath the stalk 

 of the pineal gland ; It is a narrow band of white fibres. 



The Anterior Commissure 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 is a narrow 

 hand of white fibres. 



