r 



222 A MANUAL OF ANA TO MY. 



(ft) The choroid vein collects the blood from the struc- 

 tures of the descending horn of the lateral ventricle, 

 follows along the margin of the choroid plexus to the 

 foramen of Monro, where it joins the vein above to form 

 the vein of Galen. 



(c) The vein of Galen is formed by the union of the 

 veins of the corpus striatum and the choroid plexus at the 

 foramen of Monro. It passes backward in the velum inter- 

 positum, close to the middle line, and at the posterior 

 margin of the splenium unites with a corresponding vein 

 of the other side to form the common vein of Galen, 

 which is about half an inch long and terminates in the 

 straight sinus. Besides the veins which unite to form it, 

 the vein of Galen drains the third ventricle, and the 

 posterior horn of the lateral ventricle, and the common 

 or great vein of Galen the superior surface of the cere- 

 bellum. 



DISSECTION. 



Remove the velum interpositum and the choroid plexuses, being careful 

 in doing so to leave the pineal gland behind, as the gland is firmly adherent 

 to the under surface of the posterior part of the velum. 



The Third Ventricle. Figs. 33, 34, 39, 43 to 46, 50, 51, 



54- 



The third ventricle is now exposed. Its boundaries will 

 be seen to consist of the following parts : 



Laterally, the optic thalami and the crura of the pineal 

 gland ; in front, by the anterior pillars of the fornix, the 

 anterior commissure, and the septum lucidum which fills in 

 the gap between the pillars of the fornix ; behind, by the 

 posterior commissure, the pineal gland and its diverging 

 crura ; the floor is composed of the following structures 

 already described in connection with the base of the brain, 

 (see Fig. 29) the lamina cinerea, optic chiasm, tuber cine- 



