THE THALAMUS 133 



These fiber tracts mediate an elementary but very 

 efficient form of reflex correlation without the aid of 

 specially differentiated correlation centers, as pre- 

 viously explained. 



In this scheme there are four regions where cor- 

 relations of a tolerably complex sort are effected: (i) 



Fig. 29. — Diagram of some of the conduction paths in the forebrain 

 of fishes and amphibians, based on the salamanders. For the external 

 form of such a brain, see Fig. 8, p. 73, and Herrick (1924c, Figs, i and 2). 

 Somatic sensory paths (optic, auditory, somesthetic) are drawn in heavy 

 continuous lines; thalamic radiations to the cerebral hemisphere in heavy 

 dot-and-dash lines; visceral sensory and gustatory paths in light dotted 

 lines; hypothalamic radiations to the cerebral hemisphere in light dot- 

 and-dash lines; olfactory paths in broken lines. The "striatum" includes 

 the incompletely differentiated strio-amygdaloid complex. 



the roof of the midbrain, (2) the thalamus, (3) the 

 hypothalamus, (4) the endbrain (true cerebral hemi- 

 spheres in forms above fishes). The midbrain con- 

 tains very complex correlation mechanisms which are 

 dominated physiologically by optic nervous impulses 

 coming in directly from the retina. 



The physiological characteristics of the thalamus 

 of these lower vertebrates are in some resoects very 



