THE BRAIN. 399 



vi. The infundibulum is a funnel-like depression of 

 the floor of the ventricle, about the middle 

 of its length ; to its apex the pituitary body 

 is attached. 



vii. The corpus albicans is a rounded -swelling on 

 the ventral surface of the floor of the ven- 

 tricle, in the hinder wall of the infundi- 

 bulum. 

 viii. The foramen of Monro is a vertical slit-like 

 opening at the anterior end of the ventricle, 

 between and slightly above the anterior and 

 middle commissures. It leads from the third 

 to the lateral ventricle. 



ix. The anterior pillar of the fornix is a slender 

 band of nerve-fibres in the side-wall of the 

 ventricle, running backwards and downwards 

 from the body of the fornix, beneath the 

 middle commissure, to the corpus albicans, 

 in which it ends. 



c. The mid-brain, or mesencephalon. 



i. The cavity or ventricle of the mid-brain is very 



small, 

 ii. The optic lobes, or corpora Quadrigemina, have 



moderately thick walls, and form the roof of 



the mid-brain, which is prolonged a short 



distance into them. 

 iii. The crura cerebri, which form the floor of the 



mid-brain, are very thick. 



d. The hind-brain. 



i. The fourth ventricle, or cavity of the hind- 

 brain, is moderately wide from side to side, 

 but shallow from above downwards. It has 

 a very thin roof, but thick floor and sides. 



ii. The velum medullae anterius, or valve ol Vieussens, 

 is a thin membrane extending from the optio lobes 

 to the cerebellum, and forming the roof of the anterior 

 half of the fourth ventricle. 



