156 BRAINS OF RATS AND MEN 



The physiological experiments suggest that these 

 areas are fields of preferential function, but that these 

 functions are not sharply localized in definite mosaic 

 patterns. The dispersed arrangement of the electri- 

 cally excitable points, the great variation found in 

 successive experiments in the location of these points, 

 and the nonconformity between the anatomically de- 

 fined areas and the physiologically located points, 

 suggest that the so-called motor cortex of the opossum 

 is as yet very imperfectly organized. This is further 

 supported by the very slight effect produced upon be- 

 havior by the extirpation of the entire excitable field 

 (Gray and Turner, 1924; Rogers, 1924) and by the 

 fact that the very small pyramidal tract extends only 

 as far downward as the upper end of the spinal cord 

 (Turner, 1924). These characteristics of the excitable 

 cortex of marsupials indicate that cortical control of 

 intentional movement is here in its incipience. Even 

 in man, cortical control of ordinary routine behavior 

 is probably far less precise than has been commonly 

 supposed (Franz, 1921; Franz, Scheetz, and Wilson, 



1915)- 



The brain of the rat, which will be the subject of 



several of the following chapters, has been much 



studied. The general microscopic structure of this 



brain is described and fully illustrated by Craigie 



(1925^). Most of the literature upon this subject is 



cited in the works of Donaldson (1924), Sugita (1917- 



191 8), and Craigie (i^2^a). The cerebral cortex of the 



