The Centrai, Nervous System. 



187 



with the unpaired portion of the brain (the thalamencephalon) 

 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 detached. By 

 pressing apart the tips of the hemispheres and removing the 

 pineal body the dorsal surface of the thalamencephalon will 

 be sufficiently exposed to make out the following features: 



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

 removal of the pineal body represents the dorsal portion of 

 the third ventricle (ventriculus tertius) (Fig. 53), the roof of 

 which is formed anteriorly by the superficial portion of the 

 chorioid plexus (plexus chorioideus), the latter entering the 

 ventricle at this point. 



(g) The lateral margins of the aperture are largely formed by two 

 minute spindle-shaped masses,one on either side, the habenulae. 

 Their posterior ends are united by a slender transverse band, 

 the habenular commissure (com- 



missura habenularum) . The fibres 

 of the commissure are faintly trace- 

 able forward, where they form a pair 

 of thin whitish filaments (medullary 

 striae). 



(h) The posterior commissure (commis- 



sura posterior) crosses the posterior 



portion of the roof immediately 



behind and below the habenular 



commissure, 

 (i) The walls of the third ventricle are 



formed by the thalami. They are 



broadly connected by the inter- 

 mediate mass (massa intermedia), or 



middle commissure, which may be 



seen from the dorsal surface crossing 



the cavity, 

 (j) On either side of the middle line, or 



of the habenulae, the dorsal portion 



of the thalamus forms a low, some- 

 what oval projection, the pulvinar or posterior tubercle. 



The anterior tubercle of the thalamus is a faint elevation of very- 

 small dimensions lying in the angle enclosed between the pulvinar and 

 the anterior portion of the aperture of the third ventricle. 



(k) The parts of the metathalamus are distinguishable externally 

 as two rounded projections of the lateral surfaces of the 

 thalamencephalon. One of them, the lateral geniculate 

 body (corpus geniculatum laterale), lies to the lateral side of 

 the pulvinar, and is only separated from it by a faint depres- 



Fig. 53. Diagram, showing 

 the arrangement of the parts 

 of the thalamencephalon as 

 viewed from the dorsal sur- 

 face, after removal of the 

 pineal body: a., anterior 

 thalamic tubercle; c.h., 



habenular commissure; c.p., 

 posterior commissure; c.s., 

 superior colliculus.(of mesence- 

 phalon); g.l. and g.m., lateral 

 and medial geniculate bodies; 

 h., habenula; m.i.,mas^a inter- 

 media; p., pulvinar; v.t., third 

 ventricle. 



