THE 



Aiy AXD ITS MEMBRANES. 



761 



septum lucidum), where they form the-body, but are separated from one another 

 in front and behind, forming the anterior and posterior pillars, or columnce forni- 

 cis and t-rura t\>rnicis, respectively. 



The body of the for nh- is triangular; 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, while laterally this surface forms part of the 

 floor of the body of each lateral ventricle. Its under surface rests upon the velum 

 mterpositum. which separates it from the third ventricle and from the inner por- 

 tion of the superior surfaces of the optic thalami. Its free outer edge, on each side, 

 is in contact with the choroid plexus, which projects from under it. This edge, 

 running from behind forward and inward, rests in the groove already referred to, 

 having a similar direction, on the superior surface of the thalamus, but with a 

 portion of the velum interpositum, of course, separating it from the groove. 



The anterior pillars are rounded bundles which arch downward toward the base 

 of the brain, separated from each other by a narrow interval, and each descends 



Nucleus 

 caudatus 



Beginning 

 of the post, 

 pillar $ the 

 Jornix 



Subst. nil 



Chuta 



Hippocampu 



FIG. 451. Transverse vertical section of brain behind the middle commissure. The cut-surface looks back- 

 ward, X . (Gegenbaur.) 



through the anterior portion of the optic thalamus. Each is placed immediately 

 behind the anterior commissure. At the base of the brain the white fibres of each 

 pillar make a sudden curve and form the outer part of the corresponding corpus albi- 

 <<//'. (see page 750). from which point they may be traced upward into the substance 

 of the corresponding optic thalamus. The anterior pillars of the fornix are con- 

 nected in their course with the peduncles of the pineal gland and the superficial 

 fibres of the trenia semicircularis. and receive fibres from the septum lucidum. 



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

 optic thalami an oval aperture is seen on each side : this is the foramen of Monro 

 (Fig. 442). The two openings descend toward the middle line and lead into the 

 upper part of the third ventricle. Through these openings the lateral ventricles 

 on each side communicate with the third ventricle, and consequently with each 

 other. Its boundaries are, therefore, in front, the anterior pillars of the fornix; 

 behind, the anterior extremity of the optic thalamus ; above, the body of the for- 

 nix : and below, the junction between the anterior pillars of ihe fornix and the 

 optic thalamus. 



The posterior pillars are flattened bands, and at their commencement are inti- 

 mately connected by their upper surfaces with the corpus callosum ; diverging 

 from one another, each passes downward around and behind the pulvinar of the 



