90 ANATOMY OF THE HEAD AND NECK. 



upper surface is in contact with the fornix, and its margii 

 late ."illy is the choroid plexus ; its under surface is the roc 

 of the third ventricle, and it covers the pineal body and 

 part of each optic thalamus. The velum is traversed aloiij 

 its median line by two veins called venae Galeni, whicl 

 commencing at the foramen of Monro, terminate posteriori; 

 at the straight sinus. 



The velum interpositum is now to be raised and thrown backwai 

 It must be done with care not to injure the parts beneath. 



On the under surface of the velum are the choroid plexust 

 of the third ventricle ; continuous in front by the foramei 

 of Monro with the plexus choroides of the lateral ventricle 

 which they resemble in their structure, they are conned 

 posteriorly with the pineal gland, and this connection is 

 be remembered in removing the velum, or that body m 

 be torn away with it. 



The THIRD VENTRICLE is an interval between the optic 

 thalami; its roof is the velum interpositum, and its flo< 

 the tuber cinereum at the base of the brain ; in its anteri< 

 portion are the descending crura of the fornix and the an- 

 terior commissure ; in its posterior portion the pineal gland 

 and posterior commissure, covering in the corpora quadrigc 

 mina. On its two sides we have the optic thalami, an< 

 crossing from one to the other of these, the middle or soi 

 commissure. 



The anterior commissure is a round bundle of white 

 fibres, situated just in front of the crura of the fornix, 

 which must be separated in order to display it; it crosses 

 between the corpora striata. 



The middle or soft commissure, frequently torn across in 

 examining the brain, consists of gray matter, and connects 

 the adjacent sides of the optic thalami. 



The posterior commissure is a flattened white band, con- 

 necting the optic thalami posteriori}^. 



The space between the anterior and middle commissures 

 is called the foramen commune anterius, or foramen of 

 Monro, and is the medium of communication between the 

 lateral and third ventricles, .and of the transmission of the 

 choroid plexuses and their termination, the venae Galeni. 

 This foramen is also called the Her ad infundibulum, from 

 leading down to the funnel-shaped cavity of the infundibu- 

 lum. (p. 29.) 



The space between the middle and posterior commissures 



