444 THE BRAIN 



below. Between the two layers are blood vessels and some 

 subarachnoideal trabecular tissue. The narrow, anterior end 

 of the triangular fold lies between the interventricular 

 foramina. The base is situated under the splenium of the 

 corpus callosum, and there the upper of the two layers of pia 

 mater which form the tela becomes continuous with the pia 

 mater on the corpus callosum, and the lower layer becomes 

 continuous with the pia mater on the lamina quadrigemina 

 (Fig. 1 41). 



In each of the two margins of the tela lies the chorioid 

 plexus of the central part of the corresponding lateral ventricle. 

 The plexus projects into the ventricular cavity from under 

 cover of the free edge of the fornix. Posteriorly, it is con- 

 tinuous with the part of the chorioid plexus which lies 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 

 the median junction two much smaller chorioid plexuses 

 run, backwards, in the lower surface of the tela, and project 

 downwards into the third ventricle. These are the chorioid 

 plexuses of the third ventricle (Fig. 175). 



The most conspicuous blood vessels in the tela chorioidea 

 of the third ventricle are the two internal cerebral veins 

 (O.T. veins of Galen\ which run backwards one on each 

 side of the median plane. Each internal cerebral vein 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 that vein pours its blood 

 into the anterior end of the straight sinus (Fig. 35). 



Fissura Transversa Cerebri. The name transverse fissure 

 is given to the continuous cleft through which the tela 

 chorioidea of the third ventricle and the chorioid plexuses 

 of the inferior horns of the lateral ventricles are introduced 

 into the interior of the brain. It consists of an upper or 

 middle part and two lateral parts. The middle part 

 lies between the splenium of the corpus callosum and 

 the body of the fornix, above, and the mid-brain, below. 

 The base of the tela chorioidea of the third ventricle lies 

 in it, and the blood vessels which enter and leave the tela 

 pass through it, between the layers of the tela. 



The lateral parts of the transverse fissure are the inferior 



