386 COMMISSURES OF THE BRAIN. 



The thalami nervorum opticorum being of an oval form, and 

 touching each other in the middle, there must be a vacuity 

 between them at their extremities. This vacuity is behind 

 the anterior crura of the fornix, and has been called Vulva, 

 Iter ad Infundibulnm, and Iter ad Tertium Ventriculum. 



It leads, of course, into the third ventricle ; but a passage 

 continues from it downwards and rather forwards, to the 

 infundibulum ; which is a process somewhat resembling a 

 funnel that is composed principally of cineritious substance, 

 and passes from the lower and front part of the third ventricle, 

 towards the sella turcica ; in which is situated the small body 

 called the Pituitary Gland. 



The infundibulum is hollow at its commencement, and solid 

 at its extremity near the gland. 



The adhesion of the thalami nerv. optic, to each other, at the 

 upper part of the third ventricle, has been denominated the 

 Commissura Mollis. The recession from each other at their 

 posterior extremities, in consequence of their oval figure, forms 

 another opening into the third ventricle when the fornix and 

 tela choroidea is raised, but which is closed when they are in 

 iheir natural situations upon it. 



In the back part of the third ventricle is another medullary 

 chord, called the Posterior Commissure, which appears much 

 like the anterior commissure ; but does not extend into the 

 substance of the brain in the same way. Under this chord, or 

 posterior commissure, is a passage which leads to the fourth 

 ventricle, called Iter ad Q^uartum Ventriculum, or Aqueduct 

 of Sylvius. 



Behind the third ventricle, and terminating it posteriorly, 

 are four. convex bodies, called Tubercula (^uadrigemina, or 

 Nates and Testes : the nates are uppermost and most convex ; 

 the testes are immediately below, and somewhat oval trans- 

 versely. 



The nates and testes are situated so far backwards that they 

 are near the anterior part of the upper surface of the cerebel- 

 lum, and the anterior edge of the middle of the tentorium. The 

 posterior part of the fornix is directly over them, but it 



