i6 2 THE ENGINES OF THE HUMAN BODY 



then the engine comes to a standstill, perhaps in an 

 awkward place such as a busy crossing. Hence a warn- 

 ing signal may be attached to the tank which is made 

 to sound an alarm when the stock of petrol needs 

 replenishing and keeps on attracting attention until the 

 tank is again filled. The human machine has been 

 fitted with a mechanism of this kind which we call 

 hunger. We have seen how the need for breath or air- 

 hunger is regulated by nerve centres in the medulla. 

 When the blood is lacking in oxygen or becomes charged 

 with more than its usual percentage of carbon dioxide the 

 medullary nerve centres are excited and immediately take 

 forcible control of the muscles of respiration. Apparently 

 something of the same kind happens when the available 

 supplies of fuel or of water run short in the human 

 machine. The state of the blood — so we are led to 

 suppose — sets certain nerve centres agog. In the case 

 of hunger it is a centre which is connected with the 

 stomach and has control of its movements. Prof. W. B. 

 Cannon observed that the pangs of hunger were accom- 

 panied by vigorous waves of contraction which passed 

 along the stomach. The signal for restoking in the 

 human machine is not the ringing of a bell but the 

 writhing of an empty stomach. The signal for thirst 

 is connected with, and localised in, the throat. Hunger 

 and thirst are imperious contrivances ; they are the 

 dictators of mankind ; we spend the greater part of life 

 in trying to satisfy them. In connection with the sexual 

 system of the body there is also a set of contrivances 

 of a similarly imperious kind, often challenging the will 

 for command of the human machine. 



In the mouth and throat, the first chambers or work- 

 shops connected with the preparation of fuel for the 

 human machine, Nature has worked out some of her 

 pleasantest and also some of her most cunning contri- 

 vances. The alimentary system of a motor cycle will 

 swallow a corroding acid with the same indifference as a 

 high-grade oil. That is not so with the human machine. 

 At the beginning of the human alimentary system are 



