192 TOPOGRAPHICAL ANATOMY OF THE 



tertii) is a triangular double fold of pia mater enclosing a layer of 

 aracbnoid and some blood vessels. It is interposed between the fornix 

 and part of the hippocampus on the one hand, and the thalami and the 

 third ventricle on the other. The apex of the triangle lies at the inter- 

 ventricular foramen of Monro. Each of its sides is bounded by the 

 choroid plexus of the central part of the lateral ventricle, and at its base 

 it divides into its two component sheets of pia mater that are here con- 

 tinuous with the pia covering the surface of the brain generally. The 

 ventral face of the tela carries a linear longitudinal ridge that bulges 

 towards the cavity of the third ventricle, and is produced by the choroid 

 plexus of this ventricle (plexus chorioidea ventriculi tertii). 



Cortical grey matter.\ ^-^j^^^^^p^P^^^h, /^^^^^3^^^^^ 

 White matter. ^ ^^^^i?>, S tf M ^^H^S^^^^/^^^^^^K 



-Lateral ventricle. 



-Hippocampus. 

 Anterior colliculus. 



Tegmentum.. -V.^_ .-^^^ "Aqueduct (Sylvian). 



\ ^ "'' ^ ,>^ ^ \\y ■ Central grey matter. 



Basis pedunculi. Substantia nigra. 



Fig. 90.— Transverse section of the brain at the level indicated by A in Fig. 89 



(looking forward.) 



The cleft into which the choroid tela and the choroid plexuses of the 

 inferior cornua of the lateral ventricles are insinuated is the transverse 

 cerebral fissure (fissura transversa cerebri). 



Dissection. — Seize the apex of the choroid tela with forceps and strip 

 it from the surface of the thalami upon which it rests. As this is done, 

 the narrow cleft-like third ventricle comes into view. 



Thalamus. — The thalamus ^ is an oval mass of grey matter separated 

 from the caudate nucleus by an oblique groove containing a narrow 

 white band, the stria terminaiis. When the choroid tela was removed 

 its attachment along the stria terminaiis was torn, with the result that 

 a racyo-ed edge, the tcenia chorioidea, was left. Anteriorly the stria is 



1 Thalamus [L.], OdXa/xos (thalamos) [Gr.], bedchamber (where the optic nerve 

 is born). 



