60 THE ENGINES OF THE HUMAN BODY 



however, that the gait of dark-skinned races is usually 

 easy and graceful. We Europeans, on the other hand, 

 having short heels, need more powerful muscles to move 

 them, and hence our calves are usually well developed, 

 but our gait is apt to be jerky. 



If we had the power to make our heels longer or 

 shorter at will, we should be able, as is the case in a 

 motor cycle, to alter our " speed-gear " according to the 

 needs of the road. With a steep hill in front of us we 

 should adopt a long, slow, powerful heel, while going 

 down an incline a short one would best suit our needs. 

 With its four-change speed-gear a motor cycle seems 

 better adapted for easy and economical travelling than 

 the human machine. If, however, the human machine 

 has no change of gear, it has one very marvellous 

 mechanism — which we may call a compensatory mechanism, 

 for want of a short, easy name. The more we walk, the 

 more we go hill-climbing, the more powerful do the 

 muscular engines of the heel become. It is quite 

 different with the engine of a motor cycle ; the more it 

 is used the more does it become worn out. It is because 

 a muscular engine is living that it can respond to work 

 by growing stronger and quicker. 



I have no wish to extol the human machine unduly, 

 nor to run down the motor cycle because of certain defects. 

 There is one defect, however, which is inherent in all 

 motor machines which man has invented, but from which 

 the human machine is almost completely free. We can 

 illustrate the defect best by comparing the movements of 

 the heel with those of the crank-pin of an engine. One 

 serves as the lever by which the gastrocnemius helps to 

 propel the body ; the other serves the same purpose in 

 the propulsion of a motor cycle. On referring to fig. 1 5, 

 A, the reader will see that the piston-rod and the crank- 

 pin are in a straight line ; in such a position the engine is 

 powerless to move the crank-pin until the fly-wheel is 

 started, thus setting the crank-pin in motion. Once 

 started, the leverage increases, until the crank-pin stands 

 at right angles to the piston-rod — a point of maximum 



