THE FOREBRAIN 



351 



(e) The pineal body (corpus pineale) is a small, somewhat 

 conical structure lying between the dorsal posterior tips of 

 the cerebral hemispheres (cf. p. 134). It is connected by a 

 hollow stalk with the unpaired portion of the brain (the 

 diencephalon) lying below it. The connection is concealed by 

 a mass of pigmented vascular tissue, the beginning of the 

 chorioid plexus of the third ventricle, and usually also by 

 a small portion of the dura mater containing part of the 

 sagittal venous sinus. The latter may be carefully de- 

 tached. 



By raising and pressing apart the tips of the hemispheres 

 and pulling away the pineal body with the tissue to which it 

 is attached, the dorsal surface of the diencephalon will be 

 sufficiently exposed to make out the following features: 



(/) The slit-like aperture appearing in the middle line after 

 the removal of the pineal body represents the dorsal por- 

 tion of the third ventricle (ventriculus tertius) (Fig. 116), 

 the roof of which is formed by tissue just torn away with 

 the pineal body. This roof consists of a thin membrane 

 over which lies a dense network of fine blood vessels con- 

 tained in pia mater, the chorioid plexus. Folds of the 

 membrane and plexus dip down 

 into the ventricular cavity and 

 here cerebrospinal fluid is se- 

 creted. 



(g) The lateral margins of the aper- 

 ture are largely formed by mi- 

 nute spindle-shaped masses, one. 

 on either side, the habenulae. 

 Their posterior ends are united 

 by a slender transverse band, 

 the habenular commissure 

 (commissura habenularum). 

 The fibres constituting this band 

 are faintly traceable forward, 

 where they form a pair of thin 

 whitish filaments (medullary 



Fig. 116. Diagram, showing the 

 arrangement of the parts of the 

 thalamencephalon as viewed from 

 the dorsal surface, after removal 

 of the pineal body: a., anterior 

 thalamic tubercle; c.h., habenular 

 commissiure; c.p., rostral edge of 

 posterior commissure; c.s.. superior 

 colliculus (of mesencephalon) ; g.l. 

 and g.m., lateral and medial geni- 

 culate bodies; h., habenula; m.i., 

 massa intermedia; p., lateral thal- 

 amic tubercle; v.t., third ventricle. 



