826 



THE NERVOUS SYSTEM 



conarii). These are two thin bands of white substance which extend from the 

 epiphysis anteriorly upon the thalamus, along the upper border of each lateral wall 

 of the third ventricle, and thus form the boundaries between the dorsal and mesial 

 surfaces of each thalamus. They have been called the habcnulce, from their relation 

 to the habenular nucleus, situated in the mesial grey substance below their lower 

 ends. They- are continuous across the mid-line in the habenular commissure, just 

 below the neck of the epiphysis, and between it and the posterior cerebral commis- 

 sure (figs. 582 and 613). 



The ventro-lateral surface of the thalamencephalon is continuous into the hypo- 

 thalamic tegmental region, the upward continuation of the tegmental grey sub- 

 stance of the mesencephalon. It is also adjacent to a portion of the internal cap- 

 sule. Both these relationships, as well as the fibre connections of the diencephalon 

 with the structures above and below it, are deferred until the discussion of the inter- 

 nal structure of the prosencephalon. 



The mesial surface of the diencephalon allows a better view of the shape and 

 relations of the .third ventricle. Below the line of the massa intermedia the ven- 



FIG. 615. MESIAL SECTION OF ENTIRE BRAIN, SHOWING MESIAL SURFACE OF DIENCEPHALON 

 AND TELENCEPHALON. (Alter itenle.) 



POSTERIOR 

 MPIPUfSlS 



MASSA 

 HTPOTIIAL.IVIC Sl'LCUS IXTEKXXUIA FORA.VEX IXTKRVKXTRICVLARE (MO\ROI) 



.11 Leva in' ''o/.vrx I:\/.I.II.II:M 



SKPTUM PELLUCIlirM 



SPLEXIUM OF CORPUS 

 CALLOSUJH 



AXTKRIOR .VEDUL- 

 J.ART VKLl-X 



CEREBELLUM 



FOURTH VEXTRICLE 



MEDULLA 



PONS rORPfS TUBER 



XAUMILL.VRE ClXEREl'M 



OPTIC XKRVK 

 OPTIC CUfAS.VA 



tricle is usually somewhat wider than it is along the upper margins of the thalami. 

 This greater width is occasioned by a groove in the ventromesial surface of each 

 thalamus, known as the hypothalamic sulcus (sulcus of Monro). It is along the 

 line of this sulcus that the third ventricle is continuous with the aquscductus cere- 

 bri, and thus with the fourth ventricle below, and, likewise, with the two lateral 

 ventricles of the cerebral hemispheres at its anterior end. The latter junction 

 occurs through a small oblique aperture, the interventricular foramen (foramen 

 of Monro), one into each lateral ventricle. The dorsal or upper portion of the 

 third ventricle extends posteriorly beneath its tela chorioidea (velum inierpositum) 

 to form a small posterior recess about the epiphysis. This is known as the supra- 

 pineal recess. The anterior and ventral extremity of the third ventricle involves 

 the pars optica hypothalami, which belongs to the telencephalon. 



THE TELENCEPHALON. External features. The optic portion of the 

 hypothalamus consists of that small central area of the basal surface of the telen- 

 cephalon which includes and surrounds the optic chiasma, and comprises the 



