THE MUSCLES 15 



of the machine. If it were not firmly fixed the strength 

 of the stroke of the piston would be spent in moving the 

 cylinder instead of turning the driving shaft. Firm 

 fixation is needed for the cycle engine, and it is just as 

 essential for muscle-engines. The upper end of the 

 gastrocnemius is fixed so firmly to the back of the thigh 

 bone, just above the knee-joint, by a multitude of fine 

 white fibres fastened into the bone, that even on the 

 strongest effort this fastening or attachment is never torn 

 away. The deeper part of the engine — the soleus (Plate 

 II.) — has the same kind of fastening to the back of the 

 leg bones — the tibia and fibula. Thus the engines which 

 lift the heel work from a firm base formed by the lower 

 part of the thigh bone and the upper parts of the leg 

 bones. Anatomists speak of the basis from which a 

 muscle exerts its power as its " origin," whereas they 

 name the attachment to the lever or " crank-pin " on 

 which the muscle acts, its "insertion." The crank-pin 

 on which the double-cylindered engine formed by the 

 gastrocnemius and soleus acts is the heel ; the heel is the 

 crank-pin of the foot which turns or rotates at the ankle- 

 joint. If the muscle acts when the foot is ofFthe ground 

 then it lifts up the heel, and thus tilts the front part of 

 the foot downwards. But if the foot is firmly planted on 

 the ground, as the right foot is in Plate II., then the 

 force of this engine is sufficient not only to raise the heel 

 but also to lift the weight of the body. 



In one way the muscle engine differs from a motor 

 cycle engine. If we look at the muscle of the calf of the 

 leg in Plate II. we see that it is yoked to the heel, not 

 by a rigid connecting rod like that which unites a piston 

 to its crank-pin, but by a very strong rope which is called 

 a tendon — the one going to the heel being named the 

 tendon of Achilles, because that ancient Greek warrior is 

 said to have been hung up by the heels. The heel is 

 certainly a crank-pin, and the tendon certainly serves the 

 purpose of a connecting rod, but it is tough and flexible 

 because a muscle is a " pull " engine, whereas all the 

 kinds made by man are " push " engines. A metal 



