CHAPTER XI. 

 The Diencephalon. 



AT the anterior extremity of the midbrain, the appear- 

 ance of sections changes considerably as one passes 

 forward into the next region, the 'tweenbrain, 'twixt- 

 brain, or diencephalon. At this point, the aqueduct of 

 Sylvius opens out into the third ventricle, which has the 

 form of a high, narrow slit with a rather thin floor, a mem- 

 branous roof, and an exceedingly massive wall at each side. 



The diencephalon has three main subdivisions, the epi- 

 thalamiis, a small dorsal region; the hypothalamus, a small 

 ventral region; and between these the extensive thalamus, 

 including the geniculate bodies, which are sometimes dis- 

 tinguished as the metathalamus. 



The epithalamus comprises the thin, membranous roof 

 of the third ventricle, the pineal body, and the habenulae. 

 The first of these is thrown into folds and richly supplied 

 with blood vessels forming the chorioid plexus of the third 

 ventricle. The pineal body or epiphysis is a small, unpaired 

 glandular body which projects back between the anterior 

 corpora quadrigemina, with the posterior portions of the 

 cerebral hemispheres largely covering it from the sides (PI. I.). 

 It is attached by a short stalk to the posterior portion of the 

 roof of the third ventricle, where a commissure connects the 

 habenulae. 



The habenula is a paired centre which has the form of a 

 fusiform ridge along the dorsal edge of the wall of the third 

 ventricle where the membranous roof is attached (Pis. XMIL- 

 XX., XX\T.). In section, it is seen to consist of a column of 

 gray matter covered dorsally and ventrally by layers of 



