1 6 THE ENGINES OF THE HUMAN BODY 



engine exerts its power by pushing the crank-pin, and has 

 therefore a rigid cylinder and a rigid piston-rod ; flesh 

 engines exert their power by pulling, and changing their 

 shape all the time ; they have flexible piston-rods which 

 we call tendons or sinews. Think for a moment what 

 would be the case if our bodies were fitted out with rigid 

 push engines instead of the soft, flexible ones which we 

 call muscles. Our bodies, in place of being soft and 

 supple, would be rigid and stiff as that of a tortoise. 



If we look at the walking figure shown in Plate II. we 

 see that the weight of the body is supported on the right 

 foot, and that the heel of the left foot is raised, and that 

 the left leg is just on the point of being swung forwards 

 to be planted on the ground a step in front of the right 

 foot. If our figure could actually move we should notice 

 that, just as the advanced left foot was being planted on 

 the ground the muscle-engines in the calf of the right leg 

 would come into action and raise the right heel, thus pro- 

 pelling the weight of the body forwards. 



There are three other muscle-engines fixed in the back 

 of the leg, and hidden by the gastrocnemius and soleus, 

 which are also set in motion as soon as the heel begins 

 to rise. The long tendons or piston cords of these deep 

 muscles of the leg (Plate II.) pass into the sole of the foot, 

 where they are attached to the toes and other bony parts. 

 No sooner has the heel begun to rise and the ankle to 

 bend than these three auxiliary engines exert their force in 

 steadying the foot and lifting the leg. On the outer side 

 of the leg there is placed another set of muscles — the 

 peroneal — which also helps in steadying the foot and leg 

 (Plate II.). As the heel is raised and the front part of 

 the foot is pressed on the ground a great strain falls on 

 the bones of the foot, which are fitted together so as to 

 form an arch. The arch is strained, and would yield or 

 even collapse were it not sustained by the action of the leg 

 muscles, and also by a regular battery of small muscles 

 which are placed in the sole of the foot, and help to fill 

 up the bend of its arch. Thus at the beginning of a step, 

 just as the heel is being raised, there are more than a 



