THE CEREBRUM 499 



noidal trabecular tissue. In shape this tela is triangular, and 

 the narrow anterior end or apex reaches the interventricular 

 foramen. The base lies under the splenium of the corpus 

 callosum, and there the two layers of pia mater which form 

 the tela become continuous with the pia mater on the surface 

 of the brain. 



Along each lateral margin the tela is bordered by the 

 chorioid plexus of the central part of the lateral ventricle, 

 which projects into the ventricular cavity from under cover 

 of the lateral free edge of the fornix. Posteriorly, this 

 chorioid plexus is continuous with the plexus in the inferior 

 horn of the ventricle; whilst anteriorly, it narrows greatly, 

 and becomes continuous, across the median plane, with the 

 corresponding plexus of the opposite side. From this 

 median junction two much smaller chorioid plexuses run 

 posteriorly, on the under surface of the tela, and project 

 downwards into the third ventricle. These are the chorioid 

 plexuses of the third ventricle (Fig. 205). 



The most conspicuous blood-vessels in the tela chorioidea 

 of the third ventricle are the two vencz cerebri internee 

 (O.T. veins of Galen\ which run posteriorly one on either 

 side of the median plane. Anteriorly, each is formed, at the 

 apex of the fold, by the union of the vena terminalis with a 

 large vein issuing from the chorioid plexus ; posteriorly, they 

 unite to form the great cerebral vein (O.T. vena magna 

 Galeni\ and this pours its blood into the anterior end of 

 the straight sinus (Fig. 88). 



Transverse Fissure. This name is given to the continuous 

 cleft through which the tela chorioidea of the third ventricle 

 and the chorioid plexuses of the two inferior horns of the 

 lateral ventricles are introduced into the interior of the brain. 

 It consists of an upper or intermediate part and two lateral 

 parts. The intermediate part passes anteriorly between the 

 splenium of the corpus callosum and the body of the fornix 

 above, and the roof of the third ventricle and the thalami 

 below. It is limited by the ependymal covering of the 

 chorioid plexuses, which shuts out these structures from the 

 cavity of the lateral ventricles. 



The lateral parts of the transverse fissure are the chorioidal 

 fissures. Each is continuous with the intermediate part, and 

 has been studied already in connection with the inferior 

 horn of the lateral ventricle (p. 492). 

 ii 32 a 



