160 THE HUMAN BODY 



as the other. Now let us turn the catch around so that the pin 

 must be drawn out horizontally. When the man first sees the 

 new arrangement he is able to associate the new appearance of 

 the catch with horizontal pin with his previous experience of 

 releasing the catch by pulling on the pin; he therefore opens the 

 door at the first trial. Here we have an act of reason, since the 

 association is of things not obviously related. When the monkey 

 comes to the door with rearranged fastening he finds himself con- 

 fronted with a new problem. He has no previous experience with 

 catches held by horizontal pins and he cannot associate this new 

 appearance with his previous experience of catches with vertical 

 pins. He therefore must learn this fastening as he learned the 

 other, by working at it in random fashion until he opens it acci- 

 dentally and then repeating as many of the motions of the suc- 

 cessful attempt as he can remember until he has the trick learned. 

 In all this there is no indication of reason. 



The power of reasoning is the only cerebral function possessed 

 by man and not by animals of which we have positive objective 

 proof. It is therefore the only one of which physiology can take 

 account at the present time. Physiology does not thereby deny, 

 however, the existence of many activities in the human brain which 

 are without counterpart in the brains of lower animals. While 

 associative memory accounts completely for all non-instinctive 

 actions of the lower animals, the history of the human race and 

 the experience of individuals contain much that baffles explanation 

 in terms of associative memory or of reason. The factors which 

 lead the race always onward and upward to greater and greater 

 heights of spiritual achievement are beyond the power of present- 

 day physiology to analyze or even discuss. 



Nourishment of the Brain. The cells of the cerebral cortex 

 are very dependent upon their blood supply. A slight diminution 

 in the rate of blood flow through the brain may depress the cor- 

 tical cells to such an extent that consciousness is lost. The prob- 

 lem of retaining consciousness is, then, the problem of keeping 

 the cerebral circulation up to the proper level. How this is ac- 

 complished during our waking hours, and how its falling off af- 

 fords opportunity for needed intervals of sleep will be discussed 

 in connection with the circulation of the blood (Chap. XXII). 



