192 BRAINS OF RATS AND MEN 



subcortical connections define the limits of the projec- 

 tion areas of the cortex, although these areas are not 

 so sharply delimited as in higher mammals. The rat 

 possesses very little cortex outside of these ill-defined 

 projection areas. 



Physiologically, some of the cortical fields exhibit 

 striking differences. An ill-defined fronto-parietal area 

 is electrically excitable, but this "motor" cortex and 

 the related pyramidal tracts do not seem to be essen- 

 tial for the learning and performance of many rather 

 complex acts. It is probable that tactual and kines- 

 thetic senses have cortical representation in a general 

 region which is partly but not wholly coextensive with 

 the electrically excitable field, but of the details our 

 knowledge is incomplete. Certain visual functions are 

 much more clearly related with a definite field in the 

 occipital poles. No auditory area has been physiologi- 

 cally demonstrated (Lashley, i92i<2, p. 276), though 

 comparative evidence suggests that this sense is repre- 

 sented on the lateral aspect of the hemisphere. The ol- 

 factory sense is anatomically intimately connected 

 with the archipallium, its connection with the pyri- 

 form lobe being much closer than that of any other 

 lower sensory system with the cortex. We have as yet 

 little physiological evidence of the nature of the cor- 

 tical representation of the sense of smell. 



The significance in behavior of the localization 

 pattern of the rat's cerebral cortex is further discussed 

 in the next following chapters. 



