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NEUROLOGY 



hypothalami ; and (3) the posterior part of the third ventricle. For descriptive purposes, 

 however, it is more convenient to consider the whole of the third ventricle and its 

 boundaries together; this necessitates the inclusion, under this heading, of the pars 

 optica hypothalami and the corresponding part of the third ventricle structures 

 which properly belong to the telencephalon. 



The Thalamencephalon. The thalamencephalon comprises: (1) the thalamus; 

 (2) the metathalamus or corpora geniculata; and (3) the epithalamus, consisting of 

 the trigonum habenulre, the pineal body, and the posterior commissure. 





FIG. 716. Dissection showing the ventricles of the brain. 



The Thalami (optic thalamus) (Figs. 716, 717) are two large ovoid masses, situated 

 one on either side of the third ventricle and reaching for some distance behind that 

 cavity. Each measures about 4 cm. in length, and presents two extremities, an 

 anterior and a posterior, and four surfaces, superior, inferior, medial, and lateral. 



The anterior extremity is narrow; it lies close to the middle line and forms the 

 posterior boundary of the interventricular foramen. 



The posterior extremity is expanded, directed backward and lateralward, and 

 overlaps the superior colliculus. Medially it presents an angular prominence, 

 the pulvinar, which is continued laterally into an oval swelling, the lateral geniculate 

 body, while beneath the pulvinar, but separated from it by the superior brachium, 

 is a second oval swelling, the medial geniculate body. 



The superior surface is free, slightly convex, and covered by a layer of white 

 substance, termed the stratum zonale. It is separated laterally from the caudate 

 nucleus by a white band, the stria terminalis, and by the terminal vein. It is divided 

 into a medial and a lateral portion by an oblique shallow furrow which runs from 

 behind forward and medialward and corresponds with the lateral margin of the 

 fornix; the lateral part forms a portion of the floor of the lateral ventricle, and is 



