THE CHORIOID TELA 



923 



plexus of the fourth ventricle. At the base of the tela the two chorioid plex- 

 uses join each other and then turn transversely lateral ward into the lateral re- 

 cesses of the ventricle, where they pass behind the restiform bodies and form the 

 ' comucopice.' 



The chorioid tela of the third ventricle, or velum interpositum, is a triangular 

 duplication of the pia mater which extends between the fornix above and the thai- 

 ami and third ventricle below, and in front fuses with the brain substance at the 

 interventricular foramina. 



In the transverse cerebral fissure the layers of pia forming this tela are separate, the upper 

 being the pia of the under surface of the corpus callosum and continuous with that of the ten- 

 torial surfaces of the occipital lobes; the lower being continuous into the pia enfolding the 

 epiphysis, and covering the mesencephalon, anterior medullary velum, and cerebellum. The 



Fig. 728. — Horizontal Dissectiox of the Cerebrum showixg the Tela Chorioidea of 



THE Third Ventricle. 

 (From a mounted specimen in the Anatomical Department of Trinity CoUege, DubUn.) 



The fornix has been removed to show the chorioid tela of the third ventricle. 



Veins of Galen — 1 ^ 



Crura of fornix 



Straight sinus 



Cerebellum 



Corpus 

 callosum 

 (dissected) 



Septum 

 pellucidum 



Fifth 



ventricle 



Stria termin- 

 alis of thala- 

 mus 

 Thalamus 



Chorioid tela 

 (velum inter- 

 positum) 



Chorioid 

 plexus 



Fimbria 



Hippocampus 

 major 



Collateral 



eminence 



Calcar avis 



layers forming the portion of the duplication which roofs over the third ventricle are loosely 

 adherent to each other and form the tela chorioidea proper of that ventricle. The upper surface 

 of this portion is in relation with the fornix and its lower surface, covered by the epithelial 

 chorioid lamina, hes laterally over the superior surfaces of both thalami, and mesiaUy forms the 

 roof of the third ventricle between them. The epithehum or ependyma is continuous with that 

 covering the thalami and lining the ventricles. Between the two layers of this portion, and 

 embedded in a small amount of the spongy subarachnoid tissue retained between them, are 

 the two veins of Galen, the internal cerebral veins. Posteriorly these veins unite in the region 

 of the epiphysis to form the single great cerebral vein (vena cerebri magna) which empties into 

 the straight sinus. Anteriorly the veins of Galen receive the veins of the septum pellucidum 

 from each lamina of the septum pellucidum above, and also the terminal vein (vein of corpus 

 striatum), lying in the stria terminaMs of the thalamus, empties into them from each side. 



The chorioid tela of the third ventricle or velum interpositum extends laterally 

 between the fornix and fimbria above and the stria terminalis of the thalamus be- 



