COMMUNICATION OF THE VENTRICLES. 387 



unites with the medullary matter of the cerebrum above it ; 

 there would, therefore, be a passage into the lateral ventricles 

 from behind, between the back of the fornix which is j^bove, 

 and the nates and testes which are below ; but the velum inter- 

 positum passes in from behind, and attaches the lower surface 

 of the fornix to all the parts on which it lies ; and thus closes 

 the ventricles at this place. In this membrane, immediately 

 over the posterior end of the fissure called the third Ventricle, 

 and in contact with the nates, is the Pineal Gland. This body 

 is not so large as a pea, and is formed like a pine-apple, or the 

 cone of a pine tree. 



When the fornix is raised by dissecting the membrane, it may 

 be elevated with the membrane and fornix. 



The nature of this body is not understood : it resembles a 

 small gland in its appearance, but it is very soft; and particles 

 of sand-like matter are often found in it. 



There is a small chord on each edge of the third ventricle, 

 which appears to proceed from the pineal gland, and continues 

 on the edge of the ventricle to the anterior crura of the fornix, 

 to which it unites. These chords join each other under the 

 pineal gland: they are called the pedunculi, or footstalks of the 

 pineal gland. 



The membrane connected with the pineal gland, it has been 

 said, is the tela choroidea, or velum interposition, in which the 

 plexus choroides is placed, at the edges of the fornix. This 

 membrane is extended, somewhat thinner and less vascular, so 

 as to line the surface of the ventricles. The plexus choroides 

 appears to begin at the end of each of the inferior portions of 

 the ventricles, where the pia mater penetrates from the basis of 

 the brain: it proceeds into the upper portions of the ventricles, 

 and continuing along the edge of the fornix, passes under that 

 body at its inferior angle, and meets the plexus of the opposite 

 side. Between this meeting of the plexus, and the crura of the 

 fornix, is a vacuity of an oval figure, which forms a communi- 

 cation between the ventricles of the brain. Under this vacuity 

 or foramen, the thalami nerv. optic, recede from each other, 

 and form the anterior passage into the third ventricle, described 



