4 BRAINS OF RATS AND MEN 



which float freely in the blood stream. Donaldson 

 estimates that there are about twelve thousand 

 million (12X lo^) nerve cells or neurons in the human 

 brain, and more than half of these, or ninety-two 

 hundred million (9.2X109), are in the cerebral cortex 

 (Thompson, 1899). 



Each blood corpuscle is practically an independent 

 organism. It may do its work independently of any 

 of the other cells of the body. But the most character- 

 istic thing about the nerve cells, with the related 

 nerve fibers, is that they are conductors; their most 

 significant function is to connect various parts of the 

 body so that these parts may co-operate in the 

 performance of some activity. Each of these neurons 

 has fibrous outgrowths which may connect directly or 

 indirectly with many hundreds of other neurons. 

 This is particularly true of the nerve cells of the cer- 

 ebral cortex. 



Two million blood corpuscles may do twice as 

 much work as one million; but two million cortical 

 neurons are not twice as efficient as one million such 

 elements, for the functional value of any cortical 

 structure is measured by the complexity of the inter- 

 connections of its neurons one with another, not 

 directly by the number of these elements. Two mil- 

 lion cortical neurons may provide the mechanism of 

 behavior patterns which are incomparably more com- 

 plex than those which are possible with one million 

 such cells. 



