32 THE ENGINES OF THE HUMAN BODY 



We must first find out if our muscles are supplied with 

 an explosive mixture. We can see that the blood which 

 is being pumped into them through the arteries is bright 

 red ; that is because the millions of microscopic discs or 

 red corpuscles, which float in the fluid or plasma of the 

 blood, are charged with minute loads of oxygen. In the 

 plasma there is a hydrocarbon compound — a kind of 

 sugar — which serves as fuel or petrol for the muscle. 

 Thus we find that the blood pumped into a muscle 

 through its arteries is laden with the ingredients of an 

 explosive mixture. The blood leaving by its veins has 

 lost its bright colour and is laden with waste products, 

 especially carbon dioxide. The harder a muscle is made 

 to work the quicker becomes the current of blood which 

 passes through it, and the greater is the output of 

 products of combustion. Heat also is generated. All 

 these are signs that muscles act as internal-combustion 

 engines, just as the engine of a motor cycle. But there 

 is one important difference. In the motor cycle engine 

 carbon dioxide is produced at the instant when the 

 explosion occurs and the piston gives its driving stroke. 

 This is not the case in the muscular engine. Sir Walter 

 Fletcher and Prof. F. G. Hopkins 1 found out that the 

 carbon dioxide (C0 2 ) is not thrown off" at the moment 

 of contraction but afterwards. A process of combustion 

 or oxidation therefore does take place in the muscular 

 engine, only it does not occur as an explosion but in a 

 slower and better regulated way. Yet it is clear that the 

 muscle is an internal-combustion engine of a peculiar sort, 

 very different and much superior to any kind which man 

 has yet invented. Apparently the muscular engine builds 

 up the materials supplied to it into a particular kind of 

 fuel, which it can store and use when needed. 



There is another point in which a muscular engine like 

 the biceps is greatly superior to mechanical engines. 

 With the biceps we can give what length of stroke we 

 will. We can make it move the forearm with a stroke 



1 See Croonian Lecture by Sir Walter Fletcher and Prof. F. G. Hopkins, 

 Proc. Roy. Soc, 1917, Series B, vol. lxxxix. p. 444. 



