36 THE ENGINES OF THE HUMAN BODY 



takes place. Nor can we discover any rigid wall such as 

 the cylinders of metal engines have, nor can we see any 

 sign of structures which look like pistons. All that we 

 can see is that each muscle-cylinder is made up of dark- 

 looking discs separated from each other by clear spaces or 

 discs. These alternate dark and clear discs are shown in 

 fig. 5, where the upper diagram represents a cylinder at 

 rest, the lower, one which is in a state of contraction or 

 action. When the cylinder is contracting or working the 

 discs come closer together ; they become thinner and 

 wider. When the millions of cylinders in the biceps are 

 thrown into action at the same time, as they usually are, 

 the whole muscle becomes wider, harder, and shorter ; the 

 piston cord is drawn up, and the forearm, the lever on 

 which the muscle acts, is bent. 



Now we must not forget that what we are really search- 

 ing for is the mechanism which regulates the heat of 

 muscular engines. We have just seen that a muscle, 

 such as the biceps, is made up of thousands of micro- 

 scopic cylinders, so that our problem is to find out how 

 the heating of cylinders is managed. In fig. 6 three of 

 these microscopic cylinders are drawn ; between the 

 cylinders are shown the minute kind of blood-vessels 

 or pipes called capillaries ; they are so narrow that the 

 microscopic red blood corpuscles, each carrying its load of 

 oxygen, can pass along only in single file ; their walls are 

 so thin that the fluid part of the blood, containing the 

 kind of sugar which serves as fuel for muscular engines, 

 can soak through and reach the cylinders. The cylinders 

 are also greedy for oxygen and somehow manage to 

 relieve the red blood corpuscles of their loads and send 

 back in exchange the products of combustion — or, if you 

 like, oxidation — especially C0 2 . The blood is pumped 

 into the capillaries, through very small arteries, by the 

 heart ; it is collected from the capillaries and carried away 

 by veins. There is thus a constant circulation of blood 

 round each muscle cylinder. The blood is kept at the 

 same heat as the body ; if the cylinders are above that 

 temperature they will be cooled by the blood ; if they 



