38 THE ENGINES OF THE HUMAN BODY 



seem able to regulate the rate of combustion, just as a 

 skilful driver can when he alters the throttle valve of 

 his engine. We know very well that if these nerve 

 paths are cut, then the cylinders are not only thrown 

 out of work but become broken up ; they " go to bits," 

 as it were ; they become as useless as an engine from 

 which the sparking plug has been removed. Nay, more, 

 these microscopic engine cylinders must not only be pro- 

 vided with nerves and end plates, but, to keep in good 

 repair, they must be exercised often and well. Hence 

 the need for regular exercises. No engine, except the 

 muscular one, becomes better and stronger when it is 

 given a full amount of work. 



This chapter is becoming too long, so I must again 

 ask the question which we originally set out to answer : 

 Are the muscles of the human body really internal-com- 

 bustion engines ? Can they be compared to the engines 

 of a motor cycle ? In each we have found the circulation 

 of a combustion mixture, in both heat is produced, in 

 both there are structures- which may be called sparking 

 plugs, in both there are the usual waste products of 

 combustion — water and carbon dioxide. The muscular 

 engine, therefore, seems to be an internal-combustion 

 engine of a peculiar kind. In the engine of the motor 

 cycle the space within the cylinder is lengthened when 

 an effective stroke is made, but in the muscular engine 

 the opposite happens ; the cylinder itself becomes shorter 

 and wider. It is quite clear that Nature has invented a 

 better kind of internal-combustion engine than man has 

 yet thought of. We do not know its mechanism, but 

 some day a boy, or perhaps it may be a girl, will be 

 born who will discover this great secret of Nature. 



