96 LECTURES OX THE CENTRAL NERVOUS SYSTEM. 



substance, which, from now on, is not lost sight of until we 

 reach the spinal cord. 



In the anterior part of this roof are situated the fibres of 

 the posterior commissure, and just behind these are the corpora 

 quadrigcmina. The ventricle, which is here contracted to a 

 narrow passage, extends under this roof, and that portion of it 

 which traverses the mid-brain has received the name of the 

 aqueduct of Sylvius. The entrance to the aqueduct lies just 

 under the posterior commissure. It is entirely surrounded by 

 the central gray matter. As it passes over the after-brain the 

 canal again becomes widened, and in this situation is called the 

 ventriculus quartus. Its floor is formed by the fossa rhom- 

 boidalis and its roof by the cerebellum. 



It will be well for you to study the appearance of the mid- 

 brain roof, verifying the foregoing statements on the sagittal 

 section (Fig. 56). 



Only a few words concerning the pineal gland, which, with 

 its two pedicles running along the inner surfaces of the thalami, 

 forms a part of the roof of the inter-brain. (See Fig. 11.) It 

 consists mainly of a dense mass of epithelial tubules which have 

 arisen by proliferation from the primitive fold. The pedicles 

 the pedunculi glandulte pinealis convey a few medullary "nerve- 

 fibres. These apparently originate in the ganglion habenulae 

 and (as may be demonstrated in all the lower vertebrates) in the 

 thalamus. I must again remind you of what was said in the 

 second lecture concerning the significance of the pineal gland 

 in reptiles. Besides the tubules, the pineal gland contains nu- 

 merous blood-vessels and the " brain-sand," little concretions 

 of a laminated structure, which consists mainly of calcareous 

 salts and a small amount of organic matter. 



Fig. 55 will show you the situation of the pineal gland at 

 the posterior end of the thalami and between the corpora quad- 



rigemma. 



We have hitherto had no opportunity to examine carefully 

 the base of the brain. Now that the source of some of the 



