418 FOKNIX VELUM INTERPOSITUM. 



The fornix is a white, medullary body, convex above, larger 

 behind, where it receives three roots to the hippocampus 

 major, minor , and tcenia hippocampi. The union of these 

 roots constitutes the body of the fornix, which, arching over 

 the third ventricle and resting upon the thalami, proceeds 

 forward to terminate in two white cords or pillars called 

 the anterior crura of the fornix. These descend, adhering 

 to the thalami in front, and terminating in the corpora 

 mammillaria, or albicantia. The upper surface of the fornix 

 is free and smooth ; the lower presents several oblique lines 

 called lyra, or corpus psalloides. Its structure is medullary 

 and fibrous, and its function is considered commissural, 

 forming extensive and distinct connections with different 

 parts. Its posterior crura are related to the middle and 

 posterior lobes ; its body is connected with the septum lu- 

 cidum, and corpus callosum; and its anterior crura join 

 the thalami, the peduncles of the pineal gland, the tcenia 

 semicirculares , the corpora albicantia, and the tuber cine- 

 reum. 



On the edges of the fornix is seen a fold of membrane 

 loaded with a mass of blood-vessels one on each side, and 

 formed of a reflection of pia mater, called plexus choroides. 

 This plexus enters the middle crura at the great central 

 fissure between the optic thalami and tgenia hippocampi ; 

 it is seen as a loose, floating body, following the course of 

 the hippocampus major, covering the thalami, and proceed- 

 ing forward to the anterior crura of the fornix. Behind the 

 foramen commune anterius, they both unite to form the 

 vein of Galen, vena Galeni, which runs back along the mid- 

 dle of the velum interpositum to the straight sinus. 



This Velum Interpositum (Fig. 116) is situated on the 

 under surface of the fornix. It consists of pia mater, and 

 prevents any communication between the third and lateral 

 ventricles, except at its anterior part, where is seen an 

 opening, called the foramen of Monro, beneath the anterior 

 crura of the fornix. This membrane goes back to the 

 pineal gland, and so envelops it, that there is danger of 

 removing this gland along with it, unless great care be 



