552 THE CEREBRUIVL 



brum, are attached to and embrace the under side of the corresponding 

 peduncles, and may be traced back to the thalami. Each tract is some- 

 what cylindrical towards the optic commissure, where it is connected 

 with a deposit of grey matter adjacent to the tube: cinereum. It be- 

 comes flattened and broader as it approaches the thalamus, and makes 

 a bend as it turns round the peduncle to reach the back part of that 

 body. Near this bend, which is named the knee (genu), and to the 

 outer side of the corpora quadrigemina, are placed two small oblong and 

 flattened eminences connected with the posterior extremity of the optic 

 tract. They are small masses of grey matter about the size and shape 

 of coffee-beans, placed one on the outer and one on the inner side of the 

 genu of the optic tract, and hence they are named respectively corpus 

 geniculatum externum and internum. They send fibres into the optic tract 

 and also into the thalamus of the same -side. Other fibres go directly 

 from the optic tract to the thalamus, passing between the outer corpus 

 geniculatum and the crus cerebri, and others to the anterior corpora 

 quadrigemina. 



The fibres of the optic tract are therefore derived from three 

 sources, viz. , the thalamus, the corpora quadrigemina, and the corpora 

 geniculata. 



Beneath the tract as it passes over the crus is a band of fibres pass- 

 ing in the same direction, called the " collar of the crus'' They are 

 connected behind with the optic thalamus, and in front with the tuber 

 cinereum and inner part of the corpus striatum. 



The processus a cerebello ad cerebrum, superior jjedunclcs 

 of the cereMlum, are two large white co«.-ds extending downwards 

 and somewhat outwards from the corpora quadrigemina to the fore part 

 of the cerebellum, and connecting the latter with the cerebrum. They 

 rest upon the crura cerebri, to which they are united, and between 

 them is the valve of Vieussens. Some of their fibres decussate beneath 

 the corpora quadrigemina. 



The valve of Vieussens (velum mcdullare anterius), (fig. 383) 

 stretched between the processus a cerebello ad cerebrum, is a thin layer 

 of nervous matter, which lies over the passage from the third to the 

 fourth ventricle, and, lower down, covers in a part of the fourth 

 ventricle itself. It is narrow in front, where it is connected with the 

 quadrigeminal bodies, and broader behind, where it is continuous with 

 the median portion of the cerebellum. 



Tlie valve is composed of white substance, superficial m its upper 

 portion, but concealed in its lower half by a few transverse ridges 

 of grey matter, which appear as if prolonged from the grey lamellas 

 of the cerebellum with which the valve is there continuous. Within 

 it is some grey substance, which constitutes the nucleus of the roof 

 of the fourth ventricle of Stilling, and is supposed to connect the two 

 dentate nuclei. 



From between the posterior quadrigemmal tubercles a slight median 

 ridge, named fnenulum, descends a little way upon the valve ; and on 

 the sides of this the commencing fibres of the fourth pair of nerves 

 pass transversely outwards. The back part of the valve is overlapped 

 and concealed by the superior vermiform process of the cerebellum. 



Aqueduct of Sylvius. — Beneath the corpora quadrigemina a nar- 

 row canal connects the third ventricle in front with the fourth ventricle 

 behind. It varies in shape in different parts, being T-shaped behind, 



