6 BRAINS OF RATS AND MEN 



seven million rods and cones. When the visual area 

 of the cerebral cortex is activated following the 

 stimulation of the retina by some particular field of 

 view, it is safe to say that at least a million cortical 

 nerve cells are directly stimulated by the nervous 

 impulses which discharge into this cortex from the 

 optic thalamus. These million cells activate other 

 millions, and so on in indefinite series, and the 

 aggregate of the connections actually made in any 

 extended associational process is evidently an enor- 

 mous number. This may be illustrated by some fur- 

 ther computations. 



The known arrangement of cortical intercellular 

 connections may be crudely and very conservatively 

 expressed by saying that each of the million cells 

 which we suppose to be stimulated in the visual area 

 is directly connected with ten others, each of these 

 with ten others, and so on in series. A diagram of 

 this simple geometrical progression would look like 

 a family tree in which each of a million families has 

 ten children, each of these ten, and so on. If we start 

 with the excitation of a million cortical cells, each of 

 these nervous impulses is multiplied tenfold at every 

 step in the ramification of the nerve fibers, and at the 

 fourth member of this series (the fourth generation 

 of our genealogical tree) more than the total of nine 

 thousand million cortical cells would be involved. 



But the structure is not really so simple as this. 

 The arrangement of cortical associational connections 



