132 THE CONTROL OF LIFE 



The bodily engine is able, as we have seen, to get 

 more work out of its fuel or food than any engine of 

 man's constructing, and this depends (1) on the internal 

 workshops or laboratories which prepare the food so 

 that the engines make the most of it ; (2) on the regula- 

 tions that keep up in birds and mammals a constant 

 body-temperature (between 98° and 99° Fahrenheit in 

 man) ; and (3) on the thorough arrangements for pre- 

 venting the accumulation of any form of waste. We are 

 fearfully and wonderfully made. 



There is a sort of timing or regulating system in a 

 motor-cycle, a series of revolving toothed wheels set 

 so as to be turned by the crankshaft at fixed rates, 

 but the regulation of the living body is so different that 

 Professor Keith wisely drops the comparison. Our 

 body, he says, is more like an army. There are millions 

 of millions of microscopic living units or cells, many 

 with considerable independence, yet all working with 

 considerable perfection into one another's hands and 

 so that a unified harmonious result follows. The 

 brain and spinal cord correspond to General Head 

 Quarters ; the nerves form a living telegraphic system ; 

 and there is an altogether marvellous postal system by 

 which " key-missives " are despatched throughout the 

 body. These " key-missives " (chemical messengers 

 or hormones) which are sent from ductless glands, like 

 the thyroid gland, throughout the body are compared 

 by Professor Keith to " ultra-microscopic Yale keys 

 sent out to search for the locks of letter-boxes which 

 they fit and can enter." Some kinds of key will fit 



