590 NERVOUS SYSTEM. 



albicans, from which point they may be traced upwards into the substance of 

 the corresponding thalamus opticus. The anterior crura of the fornix are con- 

 nected in their course with the optic commissure, the white fibres covering the 

 optic thalamus, the peduncle of the pineal gland, and the superficial fibres of 

 the tsenia semicircularis. 



The posterior crura, at their commencement, are intimately connected by their 

 upper surfaces with the corpus callosum; diverging from one another, they pass 

 downwards into the descending horn of the lateral ventricle, being continuous 

 with the concave border of the hippocampus major. The lateral thin edges of 

 the posterior crura have received the name corpus fimbriatum, already described. 

 On the under surface of the fornix, towards its posterior part, between the 

 diverging posterior crura, may be seen some transverse lines, and others longi- 

 tudinal or oblique. This appearance has been termed the lyra, from the fancied 

 resemblance it bears to the strings of a harp. 



Between the anterior pillars of the fornix and the anterior extremities of the 

 thalami optici, an oval aperture is seen on each side, the foramen of Monro. The 

 two openings descend towards the middle line, and joining together, lead into 

 the upper part of the third ventricle. These openings communicate with the 

 lateral ventricles on each side, and below with the third ventricle. 



Divide the fornix across anteriorly, and reflect the two portions, the one forwards, the other 

 backwards, when the velum interpositum will be exposed. 



The velum interpositum (Fig. 332) is a vascular membrane, reflected from the 

 pia mater into the interior of the brain through the transverse fissure, passing 

 beneath the posterior rounded border of the corpus callosum and fornix, and 

 above the corpora quadrigemina, pineal gland, and optic thalami. It is of a 

 triangular form, and separates the under surface of the body of the fornix from 

 the cavity of the third ventricle. Its posterior border forms an almost com- 

 plete investment for the pineal gland. Its anterior extremity, or apex, is bifid; 

 each bifurcation being continued into the corresponding lateral ventricle, behind 

 the anterior crura of the fornix, forming the anterior extremity of the choroid 

 plexus. On its under surface are two vascular fringes, which diverge from 

 each other behind, and project into the cavity of the third ventricle. These 

 are the choroid plexuses of the third ventricle. To its lateral margins are con- 

 nected the choroid plexuses of the lateral ventricles. The arteries of the 

 velum interpositum enter from behind, beneath the corpus callosum. Its veins, 

 the venae Galeni, two in number, run along its under surface; they are formed 

 by the venae corporis striati and the veins of the choroid plexuses ; the venaa 

 Galeni unite posteriorly into a single trunk, which terminates in the straight 

 sinus. 



The velum interpositum should now be removed. This must be effected carefully, especiall 

 at its posterior part, where it invests the pineal gland ; the thalami optici will then be expose 

 with the cavity of the third ventricle between them (Fig. 333). 



The thalami optici are two large oblong masses, placed between the divergin 

 portions of the corpora striata; they are of a white color superficially; in- 

 ternally, they are composed of white fibres intermixed with gray matter. Each 

 thalamus rests upon its corresponding crus cerebri, which it embraces. Ex- 

 ternally, it is bounded by the corpus striatum, and taania semicircularis; and is 

 continuous with the hemisphere. Internally, it forms the lateral boundary of 

 the third ventricle; and running along its upper border is seen the peduncle 

 of the pineal gland. Its upper surface is free, being partly seen in the lateral 

 ventricle ; it is partly covered by the fornix, and marked in front by an emi- 

 nence, the anterior tubercle. Its under surface forms the roof of the descend- 

 ing cornu of the lateral ventricle; into it the crus cerebri passes. Its posterior 

 an, I inferior part, which projects into the descending horn of the lateral ven- 

 tricle, presents two small round eminences, the internal and external gemcidate 

 tft'fies. Its anterior extremity, which is narrow, forms the posterior boundary 

 of the foramen of Monro. 



