126 BRAINS OF RATS AND MEN 



to be effected for the most part by the same neurons 

 which receive the incoming sensory radiations. As 

 seen in Figure i^A (p. 86), the axons of these 

 neurons divide and their branches may distribute to 

 widely separate cortical fields. 



The attempt has been made in the past to define 

 cerebral cortex in terms of the number of synapses 

 that intervene between the peripheral receptive sur- 

 faces and the cortex, or the number of times the 

 afferent nervous impulses have been interrupted and 

 subjected to subcortical modification through the 

 influence of other sensory systems in subcortical 

 correlation centers. But this criterion is of small 

 value, as appears from an examination of the pyriform 

 cortex which, as we have just seen, is structurally 

 well differentiated in extensive territories which are 

 reached by secondary fibers of the lateral olfactory 

 tract directly from the olfactory bulb. And I have 

 recently demonstrated a similar relation to the medial 

 olfactory tract in the case of the most anterior part 

 of the hippocampal cortex in the opossum (1924^). 

 It is clear that in these cases, notwithstanding direct 

 connection with the olfactory bulb, the olfactory func- 

 tion is one member only of a very complex system 

 and the association fibers from other cortical fields 

 play at least an equal role, so that this tissue can 

 perform true cortical functions in addition, perhaps, 

 to cortical reflexes of much simpler physiological type. 



In the higher reptiles, as in the alligator (Crosby, 



