

THE FORE-BRAIN OR PROSENCEPHALON 



841 



covered by a layer of epithelium continuous with the epithelial lining of the 

 ventricle. It extends from the interventricular foramen, where it is joined 

 ith the plexus of the opposite ventricle, to the end of the inferior cornu. The 

 part in relation to the body of the ventricle forms the vascular fringed margin 

 of a triangular process of pia mater, named the tela chorioidea of the third 

 ventricle, and projects from under cover of the lateral edge of the fornix. It 

 lies upon the upper surface of the thalamus, from which the epithelium is reflected 

 over the plexus on to the edge of the fornix (Fig. 723) . The portion in relation 

 to the inferior cornu lies in the concavity of the hippocampus and overlaps the 

 fimbria hippocampi : from the lateral edge of the fimbria the epithelium is reflected 

 over the plexus on to the roof of the cornu (Fig. 749). It consists of minute and 

 highly vascular villous processes, each with an afferent and an efferent vessel. The 

 arteries of the plexus are: (a) the anterior choroidal, a branch of the internal carotid, 

 which enters the plexus at the end of the inferior cornu; and (&) the posterior 

 choroidal, one or two small branches of the posterior cerebral, which pass forward 

 under the splenium. The veins of the choroid plexus unite to form a tortuous vein, 

 which courses from behind forward to the interventricular foramen and there joins 

 with the terminal vein to form the corresponding internal cerebral vein. 



Tail of caudate nucleus 



Choroid plexus 



Epithelial lining of ventricle 



Fimbria 

 Fimbi'iodentate 

 fissure 



Alveus 



Fascia dentata 

 hippocampi 



FIG. 749. Coronal section of inferior horn of lateral ventricle. (Diagrammatic.) 



When the choroid plexus is pulled away, the continuity between its epithelial 

 overing and the epithelial lining of the ventricle is severed, and a cleft-like space 

 is produced. This is named the choroidal fissure; like the plexus, it extends from 

 the interventricular foramen to the end of the inferior cornu. The upper part of 

 the fissure, i. e., the part nearest the interventricular foramen is situated between 

 the lateral edge of the fornix and the upper surface of the thalamus; farther back 

 at the beginning of the inferior cornu it is between the commencement of the fim- 

 bria hippocampi and the posterior end of the thalamus, while in the inferior cornu it 

 lies between the fimbria in the floor and the stria terminalis in the roof of the cornu. 



The tela chorioidea of the third ventricle (tela chorioidea ventriculi tertii; velum 

 interpositum) (Fig. 750) is a double fold of pia mater, triangular in shape, which 

 lies beneath the fornix. The lateral portions of its lower surface rest upon the 

 thalami, while its medial portion is in contact with the epithelial roof of the third 

 ventricle. Its apex is situated at the interventricular foramen ; its base corresponds 

 with the splenium of the corpus callosum, and occupies the interval between that 

 structure above and the corpora quadrigemina and pineal body below. This 





