526 NERYOUS SYSTEM. 



The body of the fornix is triangular in form ; narrow in front, broad behind. 

 Its upper surface is connected, in the median line, to the septum lucidum in front, 

 and the corpus callosum behind. Its under surface rests upon the velum interpo- 

 situm, which separates it from the third ventricle, and the inner portion of the 

 optic thalami. Its lateral edges form, on each side, part of the floor of the lateral 

 ventricles, and are in contact with the choroid plexuses. 



The anterior crura arch downwards towards the base of the brain, separated 

 from each other by a narrow interval. They are composed of white fibres, which 

 descend through a quantity of gray matter in the lateral walls of the third ven- 

 tricle, and are placed immediately behind the anterior commissure. At the base 

 of the brain, the white fibres of each crus form a sudden curve upon themselves, 

 spread out and form the outer part of., the corresponding corpus albicans, from 

 which point they may be traced upwards into the substance of the corresponding 

 thalamus opticus. The anterior crura of the fornix are connected 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 taenia semicircu- 

 laris. 



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 diverg- 

 ing posterior crura, may be seen some transverse lines, and others longitudinal 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 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 complete 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 connected the choroid plexuses of the lateral ventricles. 

 The arteries of the velum interpositum enter from behind, beneath the corpus 

 callosum. Its veins, the venas Galeni, two in number, run along its under sur- 

 face ; they are formed by the vense corporis striati, and the venae plexus choroidis ; 

 the venae 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, especially at 

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

 the cavity of the third ventricle between them (fig. 269). 



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

 portions of the corpora striata ; they are of a white color, superficially ; internally, 



