THE BIOLOGY OF HEALTH 141 



beds. But soon our metaplior breaks down, for we 

 cannot apply it to our customary experimenting with 

 ideas or to the controlling power that we have over 

 our behaviour. We are able to do ' in our head ' 

 what we do objectively in experimenting in the labora- 

 tory. 



The complexity of elements and activities in our 

 nervous system is quite unimaginable. Thus there 

 are in the convoluted part of our fore-brain (the cerebral 

 cortex) five or six times as many nerve-cells as there are 

 human beings in the world, and the complexity of inter- 

 relations is past all telling. There is a rush of multitudin- 

 ous orders always issuing from the brain, and there is 

 also a torrent of impressions — from eye, ear, nose, tongue, 

 finger-tips, skin, and the deep internal organs — that 

 flows into our central nervous system, keeping us awake 

 and conscious. All this means wear and tear, and the 

 formation of subtle waste-products, even the production 

 of heat. We have only to think of the complexity to 

 realise the importance of sleep and rest. We under- 

 stand also the value of a sleep in which the signals from 

 eye and ear have stopped. It may be a gift to be able 

 to sleep anywhere, but the better sleep is that in quiet 

 and darkness. It is also well that besides the signals 

 from eye and ear, those from the food-canal should have 

 stopped. 



Living is in some ways an extraordinarily tough 

 kind of activity ; the way our intricate, finely adjusted 

 nervous system stands treatment which we should never 

 dream of giving even to our watch, speaks volumes 



