THE FOURTH VENTRICLE. 297 



The pineal gland is a reddish, conoidal body, which 

 rests upon the upper surface of the posterior commissure 

 of the third ventricle and the tubercula quadrigemina. 

 It is invested and held in position by the velum interposi- 

 tum, and is attached to the inner upper part of the optic 

 thalami by two slender white peduncles, which conjoin 

 anterior to their attachment to the pineal gland. It is 

 formed principally of gray matter. It contains a small 

 amount of earthy salts. 



The tubercula quadrigemina, situated behind the third 

 ventricle, extends between the posterior extremities of 

 the optic thalami. It consists of two pairs of rounded 

 elevations, the upper or larger, called the nates; the 

 lower, the testes. 



They are connected with the optic thalamus by the 

 anterior and posterior commissural bands. In structure 

 they are composed principally of gray matter. The 

 aqueduct of Sylvius passes through their base ; it leads 

 from the third to the fourth ventricle. 



The fourth ventricle is situated in the upper surface 

 of the medulla. It is a diamond-shaped depression, 

 about an inch and a half long and one inch wide. It 

 is bounded above by the processus e cerebello ad testes 

 and the valve of Vieussens ; below, by the diverging 

 posterior columns and restiform bodies; and laterally, 

 at the middle, by the middle peduncles of the cere- 

 bellum. Its roof is formed by the inferior vermiform 

 process of the cerebellum. Projecting into the fourth 

 ventricle are two or three fringes of pia mater known as 

 the choroid plexuses of the fourth ventricle. 



The floor of the fourth ventricle is formed by the 

 posterior surface of the transverse commissure of the 

 gray matter of the cord and medulla exposed by the 

 divergence of the posterior columns and restiform bodies. 



