674 



THE NEEVOUS SYSTEM. 



Genu of corpus 

 callosum 



Cavura septi pellucidi 

 Septum pellucidum 

 -Caudate nucleus 



Fornix 



Column of fornix 

 Vena terminalis 



Thalamus 



Chbripid tela of third 

 ventricle 



Vena interna cerebri 



ventriculi quarti, and it is in connexion with this portion of the pia mater that the 

 chorioid plexuses of that cavity are developed. The tela chorioidea ventriculi tertii 

 (O.T. velum interpositum) is a fold of pia mater which is invaginated into the brain, 

 so that it comes to lie over the third ventricle and to project, in the shape of chorioid 

 plexuses, into the lateral ventricles. This invagination requires special notice. 



The tela chorioidea ventriculi tertii (O.T. velum interpositum) is a double layer 



or fold of pia mater which intervenes between the body of the fornix, which lies 



above it, and the epithelial roof of the third ventricle and the two thalami, 



which lie below it. Between its two layers are blood-vessels, and some subarachnoid 



-^r-^-. trabecular tissue. In shape the 



chorioid tela of the third 

 ventricle is triangular, and the 

 narrow anterior end or apex 

 reaches forwards as far as the 

 interventricular foramina. The 

 base lies under the splenium of 

 the corpus callosum, and here 

 the two layers of the tela separate 

 and become continuous with the 

 investing pia mater on the sur- 

 face of the brain by passing out 

 through a cleft called the trans- 

 verse fissure. 



Along each margin the tela 

 chorioidea of the third ventricle 

 chorioid plexus of is bordered by the chorioid 

 lateral ventricle plexus of the central part of 



the lateral ventricle, which pro- 

 jects into the ventricular cavity 

 from under cover of the free 

 margin of the fornix. It 

 should be borne in mind that 

 the epithelial lining of the ven- 

 tricle gives a complete covering 

 to the chorioid plexus. Pos- 

 teriorly the chorioid plexus is 

 FIG. 599.-DISSECTION TO SHOW THE CHORIOID TELA OF THE cont inuous with the similar 

 THIRD VENTRICLE, AND THE PARTS IN IMMEDIATE RELA- ., . P . , 



TION TO IT. structure in the interior horn 01 



the ventricle, whilst in front it 



narrows greatly, and becomes continuous across the median plane with the corre- 

 sponding plexus of the opposite side, behind the epithelial layer which lines 

 the interventricular foramen. From this median junction two much smaller 

 chorioid plexuses run backwards on the under surface of the tela chorioidea, 

 and project downwards into the third ventricle. These are the chorioid plexuses 

 of the third ventricle. 



The most conspicuous blood-vessels in the tela chorioidea are the two internal 

 cerebral veins, which run backwards, one on each side of the median plane. In 

 front, each is formed at the apex of the fold by the union of the vena terminalis 

 and a large vein issuing from the chorioid plexus ; behind, they unite to form 

 the vena cerebri magna [Galeni], and this pours its blood into the anterior end of 

 the straight sinus (Fig. 599, p. 674). 



The continuous cleft in the brain through which the chorioid tela of the 

 third ventricle and the chorioid plexuses of the inferior horns of the two 

 lateral ventricles are introduced into the interior of the brain is sometimes 

 called the transverse fissure. It consists of a superior intermediate part and 

 two lateral parts. The former passes forwards between the corpus callosum and 

 the fornix above and the roof of the third ventricle and the thalami below. 

 It is limited on each side by the epithelial covering of the chorioid plexuses, 

 which shuts out these structures from the cavity of the lateral ventricles. The 



Hippocampal 

 commissure 



Crura of fornix 

 (under surface) 



Body of fornix (thrown 

 backwards) 



