222 BRAINS OF RATS AND MEN 



Second, the cortical projection centers differ in 

 their internal structure and in the ways by which 

 they are connected with one another. These differ- 

 ences are minimal, indeed scarcely evident, in rep- 

 tiles. In the lower mammals they are more evident. 

 The olfactory cortex (archipallium) is sharply set off 

 from the rest, and its various parts have attained 

 highly specialized organization. The neopallium is 

 much more uniform in structure and doubtless also in 

 function. Yet it is not equipotential; there is obvious- 

 ly a preferential relation of each ill-defined projection 

 field with some particular lower sensori-motor sys- 

 tem. 



In lower mammals the various parts of the cortex 

 are much more closely knit together by associational 

 fibers than in any reptiles, as shown by the enormous 

 increase in the underlying white matter. This is 

 scarcely visible in reptiles; in even the most primi- 

 tive mammals it is very massive. This implies that 

 the mammalian cortex is much better adapted to act 

 as a whole, and in its simpler reinforcing action one 

 part seems to be about as good as another, as Lash- 

 ley's experiments demonstrate. But the limits of this 

 equipotential facilitation are quickly reached, and the 

 differential functions of the parts come into play in 

 most normal cortical activities. 



In the smaller marsupials and rodents the cortical 

 charts already presented show that most of the neo- 

 pallium is occupied by the projection centers, while 



