351 Dl?. G. ELLIOT SMITH ON 



and the nieseuccphalon may be seen cropping out in a \ie\\ of the ventral surface of the 

 hraiu. In the angle between the lateral part of the pons Varolii and the crus cerebri we 

 find a triangular area which we may call the area tegmenti, because it is the surface of 

 the iegmerdum, i. e. the essential part of the basal region of the mesencephalon. In 

 Man the corresponding region is called the area lemnisci, because an important nerve- 

 tract in the tegmentum known as the fillet or lemniscus appears on the surface in this 

 situation. In front of the area tccpnenti we find a very prominent hemispherical boss of 

 grey substance, the corjms genicidatum postlcum [vel mediale']. This body is partly 

 hidden from view by the overlapping fold of cortex (pyriform lobe). If we remove this 

 cortical operculum we shall be able to see the optic tract extending obliquely backward 

 and outward across the crus cerebri, and then expanding to cover a small ill-defined boss 

 of grey substance Avliich is placed just in front of the posterior geniculate body. This is 

 the corpus cjeniculatum. anticum [vel lalcrale]. Id a view of the base of the brain with 

 the cerebral cortex undisturbed, the anterior geniculate body and the lateral part of the 

 optic tract are hidden from view. 



Pis. 27. 



colum. fornicis 



.cortex 

 Corp. striat. 



gang], habenul. 



thalam. opt. 

 — -hippocamp. 



-Corp. pineale 

 'corp. geniculat. post. 

 r--corp. quadrigem. 

 vel. meduUare 

 --nerv. V. 

 ped. cerebel. sup. 

 '" colum. cerebel. 

 ^-nerv.Vin. 



"tuber, acust. lat. 

 ~'- tuber, acust. med. 

 "•-ventr. IV. 



Dissection to expose dorsal .surface of brain-stem in Ori/cterojiKs. Enlarged .| diam. 



In a profile view of this region (fig. 2) we can better appreciate the large dimensions 

 of the prominent posterior geniculate body, which seems to rest on the crus cerebri, and 

 by contrast Ave can see how ill-defined are the boundaries of the anterior geniculate body , 

 which appears as a slight swelling that can only with difficulty be distinguished from 

 the optic tract in front and the optic thalamus on the dorsal side. Above and behind the 

 posterior geniculate body we see the corpora geniculata in profile. But these bodies can 

 be more satisfactorily studied in a dorsal view (fig. 27). 



The dorsal siu-faces of the optic thalami and the anterior pair of the corpora quadri- 



