THE THALAMUS 141 



systems to concentrate in particular parts of the 

 nucleus difFusus. There are no specific "nuclei," but 

 certain parts of the thalamus receive more fibers from 

 one source and other parts from different sources. In 

 other words, one functional system may be prepo- 

 tent in one place and a different system in another 

 place. The influence which the thalamus may exert 

 upon behavior at any particular moment will depend 

 upon the balance or physiological equilibrium among 

 all the incoming nervous excitations then prevailing in 

 the thalamus as a whole. 



In the frog the splitting up of the nucleus diffuses 

 thalami into local "centers,'' each with characteristic 

 and tolerably well-defined functional connections, is 

 further advanced, and in all higher animals most of 

 the neurons of the thalamus are arranged in definite 

 "nuclei." The number and arrangement of these 

 nuclei vary from species to species. In man more than 

 two dozen such local centers can readily be distin- 

 guished microscopically in the thalamus. 



The thalamic nuclei are interconnected by a com- 

 plicated fabric of nerve fibers, some arranged in defi- 

 nite bundles or tracts and some diffusely spread. The 

 entire thalamus, even in man, must therefore be 

 thought of as activated locally niore or less intensely 

 in a variety of places under the influence of processes 

 going on in other parts of the brain. These local ex- 

 citations spread more or less widely, in part through 

 definite pre-established fiber tracts and in part dif- 



