108 AXATOMY OF THE NeRVOUS SYSTEM 



many dliYerent kinds of afferent impulses, both visceral and 

 somatic. 



The remaining mass which is usually classified with the 

 basal nuclei is the claustrum, a thin plate of gray matter situ- 

 ated between the putamen and the overlying cortex (insular 

 area). It is separated from the putamen by a rather thin 

 layer of the white matter of the cerebral hemisphere, the 

 external capsule {capsula externa), and from the cerebral 

 cortex by a considerably thinner layer, the capsula extrema. 

 The latter is practically absent in the rat, so that the claustrum 

 cannot readily be distinguished in Weigert sections (Pis. XX.- 

 XXII.). It can be recognized fairly easily, how^ever, in 

 preparations which show the cell-structure, though even there 

 its limits are not very clear, tending to blend with the deepest 

 layer of the adjoining cortex. In the rat, as in some other 

 lower mammals, its ventral edge extends a short distance below 

 the rhinal fissure. The claustrum has been shown by Elliot 

 Smith to be an infolded portion of the cerebral cortex, a 

 source from which the putamen and the greater part of the 

 caudate nucleus have also been derived. Both the fibre- 

 connections and the function of the claustrum are still un- 

 known. 



The neopallium, the non-olfactory cerebral cortex with 

 its subjacent white matter, makes up the dorso-medial, the 

 dorsal, and most of the lateral surfaces of the hemisphere 

 of the rat. The early steps of its evolutionary expansion 

 were briefly reviewed in the last chapter. The same process 

 of expansion has continued through the mammalian series 

 to reach its culmination in man. This part of the brain is 

 the physical substratum of all the finer forms of consciousness, 

 all sensory discrimination, and all those processes which we 

 know as mental, intellectual, or psychic.^ Experiments with 



^The physical basis of the emotions, however, is in the thalamus 

 (Head). 



