THE THIRD VENTRICLE. 



the corpus callosum and fornix, below by the tubercula 

 quadrigemina and optic thalami, and laterally and above 

 on each side by the corpus fimbriatum of the middle cornu. 

 The septum lucidum is the vertical partition between the 

 lateral ventricles. It is attached above along the median 

 line of the under surface of the corpus callosum, below 

 along the median line of the upper surface of the fornix. 

 It consists of two laminae of white and gray matter 

 inclosing a small serous sinus, the fifth ventricle. 



Immediately below the fornix is the velum inter- 

 positum, a process of pia mater transmitted by the 

 transverse fissure. It is triangular in shape, and pre- 

 sents along its free borders the choroid plexus. Two 

 similar tufted choroid plexuses hang down from its 

 under surface into the third ventricle. Running longi- 

 tudinally along its upper surface are two large veins, 

 the venae galeni, which receive the blood from the 

 choroid plexuses, and also from the corpora striata. 

 They empty into the straight sinus. 



The third ventricle is located between the two optic 

 thalami. It is bounded above by the fornix, from which 

 it is separated by the velum interpositum ; laterally, by the 

 inner surfaces of the optic thalami and the peduncles of 

 the pineal gland ; the floor, by the structures of the inter- 

 peduncular-space at the base of the brain; in front, by 

 the anterior pillars of the fornix ; behind, by the anterior 

 aspect of the corpora quadrigemina and the aqueduct of 

 Sylvius. The third ventricle is lined by gray matter, 

 and is crossed by three commissures, the anterior, 

 white, placed in front of the anterior pillars of the for- 

 nix, passes between the corpora striata ; the middle, 

 gray, passes between the optic thalami ; and the poste- 

 rior, white, between the optic thalami ; it supports the 

 pineal gland. Four openings lead into the third ventricle, 



