34 THE ENGINES OF THE HUMAN BODY 



over-heating or over-cooling will stop their engines, but 

 have also discovered that there is a certain temperature at 

 which they go at their best. No one has discovered the 

 means by which a metal engine can be kept at the right 

 temperature, but Nature has ; she has fitted out muscular 

 engines with a mechanism which keeps them constantly 

 near the temperature best suited to bring out their work- 

 ing powers. That degree of heat is the temperature at 

 which the living body is constantly maintained — about 

 98 Fahrenheit. Muscular engines never become over- 

 heated and rarely over-cooled. Indeed, athletes who run 

 races, and men who box or fence, know very well that 

 their muscles work best when they are a little over- 

 heated by active exercise. 



We are now to examine this wonderful mechanism 

 which regulates the heat of muscular engines, but before 

 we do that I must make a confession. I have been 

 speaking of the biceps muscle as if it were an engine with 

 a single cylinder. Now, although the biceps acts as a 

 single engine it is made up of myriads of cylinders. In 

 the well-developed biceps of a working-man there are 

 600,000 of these microscopic engine cylinders. Only 

 medical students dissect the human biceps muscle and 

 examine it closely, but nearly every one is familiar with 

 the structure of muscular engines because we live on 

 them. We are all engine-eaters, except vegetarians. We 

 eat the engines which gave the power of movement to 

 some self-complacent ox, sheep, or pig. Their red flesh 

 or muscle, especially if it has been boiled, can be separated 

 into very fine threads or bundles. To see the cylinders — 

 or primitive fibres, as the anatomist names them — we 

 must take from a boiled muscle one of these very small 

 threads, and then tease it out under water with fine needles 

 until the shreds are so minute that they can scarcely 

 be seen with the naked eye. When such shreds are 

 examined through a compound microscope they are seen 

 to be made up of narrow soft columns or cylinders, each 

 of them being wrapped round by a transparent skin or 

 covering. How small these cylinders are may be realised 



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