LATERAL VENTRICLE MIDDLE CORNU. 525 



entire length of the floor of the middle horn of the lateral ventricle. At its lower 

 extremity it becomes enlarged, and presents a number of rounded elevations with 

 intervening depressions, which, from presenting some resemblance to the claw of 

 an animal, is called the pes hippocampi. If a transverse section is made through 

 the hippocampus major, it will be seen that this eminence is the inner surface of 

 the convolution of the corpus callosum, doubled upon itself like a horn, the white 

 convex portion projecting into the cavity of the ventricle ; the gray portion being 

 on the surface of the cerebrum, the edge of which, slightly indented, forms the 

 fascia dentata. The white matter of the hippocampus major is continuous through 

 the corpus fimbriatum, with the fornix and corpus callosum. 



The pes accessor ius or eminentia collateralis has been already mentioned, as a 

 white eminence, varying in size, placed between the hippocampus major and 

 minor, at the junction of the posterior with the descending cornu. Like the hippo- 

 campi, it is formed of white matter corresponding to one of the sulci, between 

 two convolutions protruding into the cavity of the ventricle. 



The corpus fimbriatum is a continuation of the posterior pillar of the fornix, 

 prolonged, as already mentioned, from the central cavity of the lateral ventricle. 



Fascia dentata. On separating the inner border of the corpus fimbriatum from 

 the choroid plexus, and raising the edge of the former, a serrated band of gray 

 substance, the edge of the gray substance of the middle lobe, will be seen beneath 

 it ; this is the fascia dentata. Correctly speaking, it is placed external to the 

 cavity of the descending cornu. 



The transverse fissure, is seen on separating the corpus fimbriatum from the thal- 

 amus opticus. It is situated beneath the fornix, extending from the middle line 

 behind, downwards on either side, to the end of the descending cornu, being 

 bounded on one side by the fornix and the hemisphere, and on the other by the 

 thalamus opticus. Through this fissure the pia mater passes from the exterior of 

 the brain into the ventricles, to form the choroid plexuses. Where the pia mater 

 projects into the lateral ventricle, beneath the edge of the fornix, it is covered by 

 a prolongation of the lining membrane, which excludes it from the cavity. 



The septum lucidum forms the internal boundary of the lateral ventricle. It 

 is a thin, semi-transparent septum, attached, above, to the under surface of the 

 corpus callosum ; below, to the anterior part of the fornix ; and, in front of this, 

 to the prolonged portion of the corpus callosum. It is triangular in form, broad 

 in front, and narrow behind, its surfaces looking towards the cavities of the ven- 

 tricles. The septum consists of two lamina, separated by a narrow interval, the 

 fifth ventricle. 



Each lamina consists of an internal layer of white substance, covered by the 

 lining membrane of the fifth ventricle ; and an outer layer of gray matter, covered 

 by the lining membrane of the lateral ventricle. The cavity of the ventricle is 

 lined by a serous membrane, covered with epithelium, and contains fluid. In the 

 foetus, and in some animals, this cavity communicates, below, with the third 

 ventricle, but in the adult, it forms a separate cavity. In cases of serous effusion 

 into the ventricles, the septum is often found softened and partially broken 

 down. 



The fifth ventricle may be exposed by cutting through the septum and attached portion of the 

 corpus callosum, with the scissors ; after examining which, the corpus callosum should be cut 

 across, towards its anterior part, and the two portions carefully dissected, the one forwards, the 

 other backwards, when the fornix will be exposed. 



The fornix is a longitudinal lamella, of fibrous matter, situated beneath the 

 corpus callosum, with which it is continuous behind, but separated from it in front 

 by the septum lucidum. It may be described as consisting of two symmetrical 

 halves, one for either hemisphere. These two portions are joined together in the 

 middle line, where they form the body, but are separated from one another in 

 front and behind ; in front, forming the anterior crura, and behind, the posterior 

 crura. 



