258 BRAINS OF RATS AND MEN 



ologically isolated from the reflex centers and the 

 great lines of through traffic of the brain stem, and the 

 degree of this detachment is one of the factors affect- 

 ing the differentiation of the cortex. 



Reflex responses usually follow immediately upon 

 presentation of their adequate stimuli, and their cen- 

 tral adjusting mechanisms are elaborated within the 

 relatively direct lines of conduction employed (me- 

 dulla oblongata, midbrain, thalamus, corpus stria- 

 tum, etc.). But cortical functions are in larger meas- 

 ure delayed reactions and individually acquired con- 

 trols redirecting and recombining the innate reflex 

 and instinctive patterns. The introduction of large 

 masses of higher correlation tissue concerned with de- 

 layed reactions within the innate reflex apparatus 

 would tend to distort the reflex patterns and inter- 

 fere with their prompt and efficient independent ac- 

 tion when this is desirable. The addition of associa- 

 tional tissue outside of these deeper centers leaves 

 their local activities unimpaired no matter how ex- 

 tensively the overlying cortical fields are developed, 

 a point that has been called to my attention by Pro- 

 fessor Landacre. 



This detachment of the cortex may be compared 

 with that of a large switchyard of a great railroad 

 center which is set off to one side of the main lines of 

 through traffic. Where many trunk lines of railroad 

 intersect switch tracks must be provided for the 

 through trains which pass from one Hne to another, 



