82 BRAINS OF RATS AND MEN 



in fishes and amphibians are most evident. The 

 thalamic centers of sight, hearing, and cutaneous 

 sensibility are large and well separated from one 

 another; and from the thalamus massive tracts run 

 forward into the lateral wall of the cerebral hemi- 

 sphere. The olfactory organs and cerebral centers of 

 all reptiles are large, but the same is true of most 

 lower forms, and the increment in size and complexity 

 of internal structure of the reptilian hemisphere is 

 to be explained in terms of its more extensive con- 

 nections with the thalamus, which in turn is corre- 

 lated with the more diversified adjustments which 

 these animals make to their surroundings in air and 

 earth and water. 



The fibers which ascend from the thalamus to the 

 cerebral hemisphere are known as the "thalamic 

 radiations," and the large increase in sensory dis- 

 charge through these fibers into the lateral wall of 

 the reptilian hemisphere involves a corresponding 

 increment in the hemisphere itself; the larger volume 

 of business to be transacted requires more commodi- 

 ous housing. This increased business is of two sorts, 

 for which two closely related offices are organized: 

 first, a notable elaboration of the reflex and instinctive 

 behavior for which the corpus striatum complex is 

 reorganized on a larger scale; and, second, the dif- 

 ferentiation of the cerebral cortex related in some way 

 not clearly understood to more complex behavior 

 patterns. These cortical capacities in reptiles, though 



