228 C. JUDSON HERRICK 



Starting from the very primitive "somatic area" of fishes, we 

 can trace in amphibians, reptiles, birds and mammals the grad- 

 ual differentiation of the non-olfactory parts of the cerebral 

 hemispheres. v In the initial stages of its evolution this structure 

 is organized much like the thalamus. It receives no afferent 

 fibers which come directly from any simple primary sensory 

 center, but only fibers from association centers of the second 

 or third order, which are themselves capable of elaborating 

 complicated reflex responses. 



The thalamus, as we have seen, has its own intrinsic system 

 of association centers, which discharge downward into the cere- 

 bral peduncles, and this is the primary reflex apparatus of this 

 part of the brain. The thalamo -cortical connections arose to 

 prominence later in the evolutionary history, though feeble rudi- 

 ments of these are present in lower brains. Parallel with the 

 enlargement of these cortical connections a special part of the 

 thalamus was set apart for them and from the Amphibia upward 

 in the animal scale this dorsal part, of the thalamus assumed 

 increasingly greater importance. This part is termed by Edinger 

 the neothalamus and makes up by far the larger part of the 

 thalamus in the human and all other mammalian brains. It 

 occupies the dorsal part of the thalamus proper and comprises 

 most of the great thalamic nuclei (anterior, lateral and ventral 

 nuclei pulvinar and lateral and medial geniculate bodies). The 

 primitive intrinsic reflex thalamic apparatus in man is a rela- 

 tively unimportant area of medial grey matter and the sub- 

 thalamic region (corpus Luysii, lattice nucleus, etc., not to be 

 confused with the hypothalamus which lies farther down in the 

 tuber cinereum and mammillary bodies). 



The neothalamus, accordingly, serves as a sort of vestibule 

 to the cortex, every afferent impulse from the sensory centers 

 (except the olfactory system) being here interrupted by a sy- 

 napse and opportunity offered for a wide range of subcortical 

 associations. The olfactory cortex (hippocampal formation) has 

 a similar relation to subcortical correlation centers in the olfac- 

 tory area in the anterior perforated space, septum, etc. 



From these anatomical considerations it follows that no sim- 

 ple sensory impulse can, under ordinary circumstances, reach 

 the cerebral cortex without first being influenced. by subcortical 

 association centers, within which complex reflex combinations 



