202 DISSECTION OF THE BRAIN. 



The inner boundary (septum ventriculorum) is a thin layer which is 

 sometimes named septum lucidum (fig. 63, b). Its extent corresponds 

 with the central part of the corpus callosum. Below it and the fornix, 

 opposite the front of the optic thalamus, is the aperture of communication 

 (foramen of Monro) between the two lateral ventricles. 



The lower or descending part of the ventricle winds beneath the optic 

 thalamus, and forms a curve like the half-bent forefinger. The roof is 

 formed by the optic thalamus and the contiguous part of the hemisphere. 

 In the floor is a large curved, convex eminence, somewhat indented at 

 the end the hippocampus major (/) ; and along its concave margin is a 

 thin white band taenia (cT), which is prolonged from the fornix. Ex- 

 ternal to the projection of the hippocampus is another white eminence, 

 the eminentia collaterals (&), which tapers from above down. In this 

 part of the ventricle is the vascular fringe of the plexus choroides. 



The septum lucidum (fig. 63, b), or the thin structure between the 

 lateral ventricles, is translucent, and hangs vertically in the middle line 

 along the anterior two-thirds of the corpus callosum. It is somewhat 

 triangular in form, with the larger part turned forwards, and the pointed 

 extremity backwards. Its surfaces look to the lateral ventricles. The 

 upper border is attached altogether to the under surface of the corpus cal- 

 losum ; and the lower border is joined in part to the fornix (c), but in 

 front of that body it is inserted into the under or prolonged portion 

 (rostrum) of the corpus callosum. The septum consists of two layers, 

 which inclose a space the fifth ventricle ; and each layer is formed of 

 white substance, with an external coating of gray matter. 



Dissection. The space of the fifth ventricle will come into view by 

 cutting through the part of the corpus callosum which remains in the 

 middle line, and by detaching the anterior half from the septum lucidum, 

 and raising it. (See fig. 63.) 



The ventricle of the septum, or the fifth ventricle, is a triangular space 

 in the fore part of the ventricular partition, where the depth is greatest. 

 Like the septum containing it, its largest part is in front. Its surface has 

 an epithelial covering like that in the lateral ventricles. In the adult it 

 is closed ; but in the foetus it opens interiorly into the third ventricle be- 

 tween the pillars of the fornix. 



Dissection. The fornix is to be next examined. To lay bare this 

 body the posterior part of the corpus callosum should be detached with 

 care from it, and thrown backwards ; and the septum lucidum should also 

 be removed from its upper surface. 



The fornix, or arch (fig. 63, c), is a thin white horizontal stratum be- 

 neath the corpus callosum, which, projecting on each side into the lateral 

 ventricle, forms part of the floor of that cavity. Its central part or body 

 is triangular in shape, with the base turned backwards ; and it is continu- 

 ous with the rest of the brain by processes or crura before or behind. 



To the upper surface of the body, along the middle line, the septum 

 lucidum is attached. Each border is free in the corresponding lateral 

 ventricle, where it rests on the optic thalamus ; and along it lies the cho- 

 roid plexus. At its posterior part it joins the corpus callosum in the 

 middle line, whilst on each side it sends off a small riband-like band 

 taenia hippocampi (rf), along the concave margin of the hippocampus 

 major. At the anterior part it is arched over the foramen of Monro, 

 opposite the front of the optic thalamus, and ends likewise in two pro- 



