56 BRAINS OF RATS AND MEN 



no food is presented. The neurologic mechanism in- 

 volved in its simplest possible form is shown in the 

 diagram by the broken or dash lines. From the 

 primary gustatory center a path ascends to a higher 

 correlation center, thence by a return motor path to 

 the salivatory nucleus, so that in the "natural" re- 

 action the higher center may be involved, as well as 

 the medulla oblongata. From the primary auditory 

 center, also, a pathway ascends to the correlation 

 center. The "natural" return path for this system 

 leads to the spinal center for turning the head. But 

 within the correlation center there are various col- 

 lateral paths (drawn as fine dotted lines). 



The centers and definite fiber tracts employed 

 in the natural reflex behavior are laid down in the 

 innate structural organization. These pathways are 

 taken by preference in all ordinary reflex behavior. 

 The collateral connections indicated in the diagram 

 by the fine dotted lines spread out difl^usely through 

 the correlation center without forming definite fiber 

 tracts between specific functionally defined regions. 

 The result is that the activation of the fixed innate 

 reaction mechanisms results also in an irradiation of 

 nervous excitation throughout the correlation center 

 as a whole. This may lower the threshold of all 

 parts of the center for any other kind of excitation. 



In the early stages of training the excitation of the 

 auditory system by ringing the bell does not of itself 

 cause salivation because there is no preformed path- 



