548 THE CEREBRUM. 



descending cornu on one side, over the constricted part of the cerebrum, to 

 the extremity of the descending cornu of the other side. It is bounded 

 above by the corpus callosum and fornix in the middle, and more externally 

 on each side by a free margin of the hemisphere : inferiorly it is bounded 

 near the middle line by the corpora quadrigemina, and on each side by the 

 cms cerebri and posterior part of the optic thalamus. 



In the free margin of the hemisphere brought into view by opening out the 

 part of the transverse fissure which leads into the descending coruu of the 

 lateral ventricle, there are seen (1st) the ribbon-like ledge formed by the 

 corpus fiinbriatmn, internally to the hippocampus major ; (2nd) beneath 

 this, a small grey indented ridge, the fascia dentata ; and (3rd) beneath the 

 fascia dentata, the gyrus hippocampi. On making a transverse section, it is 

 seen that the corpus fimbriatum is the free margin of the white substance of 

 the hemisphere, and that the fascia dentata is the free margin of the cortical 

 substance, and is continuous with the grey matter of the hippocampus 

 major, and that thus the hippocampus major is the swelling in reverse of 

 the sulcus between the fascia dentata and gyrus hippocampi. The fascia 

 dentata can be traced up to the pad or bourrelet : its upper part is free of 

 dentations, and is sometimes named fasciola cinerea. The dentations cor- 

 respond with blood-vessels passing to and from the choroid plexus. 



The velum interpositum or tela choroidea, the membrane which connects 

 the choroid plexuses of the two sides together, is a prolongation of the pia 

 mater through the transverse fissure. It corresponds in extent with the 

 fornix, which rests upon its upper surface ; and its more highly vascular 

 free borders, projecting into the lateral ventricles, form the choroid plexuses. 



The choroid plexuses appear like two knotted fringes, reaching from 

 the foramen of Monro, where they meet together beneath the fornix, to the 

 point of each descending cornu. They consist of a highly vascular villous 

 membrane. The villi with which they are covered are again divided upon 

 their surfaces and at their borders into small processes, along which fine 

 vessels are seen to run, Numerous small vessels pass between the plexuses 

 and the surface of the corpora striata, as well as other neighbouring parts, and 

 the epithelium of the ventricles is continued over their surface. Thus it is 

 only at the foramen of Monro that the epithelial lining of the lateral ven- 

 tricles is continuous with that of the third ventricle. 



The epithelium changes its character where it covers the plexus. It is there 

 composed of large spheroidal corpuscles, in each of which is seen, besides a distinct 

 nucleus, several yellowish granules, and one or more dark round oil-drops. According 

 to Henle each of these cells is provided with short, slender, acuminate, transparent, 

 and colourless processes. 



On raising the velum interpositum, two slight vascular fringes are seen 

 running along its under surface, and diverging from each other behind. 

 They form the choroid plexuses of the third ventricle. 



The choroid artery enters the velum mterpositum at the point of the 

 descending cornu ; and other arteries enter from behind, beneath the corpus 

 callosum. The greater number of the veins terminate in two principal 

 vessels named the veins of Galen, which run backwards on the velum mter- 

 positum, and passing out beneath the corpus callosum pour their blood into 

 the straight sinus, having generally first united into a single trunk. 



Bichat supposed that the arachnoid membrane entered the third ventricle in the 

 form of a tubular process, which passed beneath the posterior end of the corpus 

 callosum and fornix, through the velum interpositum, and thus opened into the 



