DiEXCEPHALOX 77 



tract (Pis. XII. -XM., XXM.). This fasciculus is considerably 

 smaller and less conspicuous than the mamillo-thalamic tract. 

 It probably conveys effector impulses which are perhaps 

 concerned with the act of sniffing. 



Anterior to the mamillary bodies, is more gray matter, 

 which forms the tuber cinereum, and from which there pro- 

 jects ventrally the hollow stalk of the hypophysis, the in- 

 fundibulum (Pis. X\\-X\1II.). The tuber cinereum, like the 

 mamillary body, receives olfactory and other fibres, and these 

 two regions together constitute an important olfactory corre- 

 lation area, which is probably chiefly olfacto-visceral in func- 

 tion. The mamillo-tegmental tract leads the resultant nerve 

 impulses down towards the motor centres, while the mamillo- 

 thalamic tract conducts similar impulses to the anterior 

 thalamic nucleus, where they are further correlated with 

 others of somatic origin. 



Thus we have in the epithalamus olfacto-somatic corre- 

 lation, in the hypothalamus olfacto-visceral correlation, and 

 in the anterior thalamic nucleus olfacto-viscero-somatic corre- 

 lation. 



The thalamus is made up of dorsal and ventral parts, of 

 which the former is very much larger in mammals than is 

 the latter. 



The ventral part is the subthalumus , which has sometimes 

 been confused with and sometimes included in the hypo- 

 thalamus. Its phylogenetic history, however, seems to show 

 that it should be regarded as a ventral part of the thalamus, 

 of which it is the motor coordination centre. 



The subthalamus is the direct forward continuation of 

 the substantia nigra of the midbrain and the reticular for- 

 mation just dorsal to it. It is not so large in the lower mam- 

 mals as in man and the primates, though this fact does not 

 strike the observer on account of the still greater enlargement 

 in the latter forms of the dorsal part of the thalamus. The 

 ventral region is by no means simple, though not very large, 



