68 THE ENGINES OE THE HUMAN BODY 



In the human machine the levers of the arm have 

 been fashioned, not for climbing, but for work of another 

 kind — the kind which brings us a livelihood. We must 

 have perfect control over our hands ; the longer the 

 lever of the forearm is made, the more difficult does 

 control of the hand become. Hence in the human 

 machine the forearm is made relatively short and the 

 upper arm long. 



We have just seen that the brachial muscle could at 

 one time move the forearm and hand, but that when they 

 are fixed it could then use the humerus as a lever and 

 thereby lift the weight of the body. What should we 

 think of a metal engine which could reverse its action 

 so that it could act through its piston-rod at one time 

 and through its cylinder at another ? Yet that is what 

 a great number of the muscular engines of the human 

 machine do every day. 



There is another little point, but an important one, 

 which 1 must mention before this chapter is finished. 

 1 have spoken of the forearm and hand as if they formed 

 a single solid lever. Of course that is. not so ; there are 

 joints at the wrist where the hand can be moved on the 

 forearm. But when a weight is placed in the hand these 

 joints become fixed by the action of muscles. The fixing 

 muscles are placed in the forearm, both in front and 

 behind, and are set in action the moment the hand is 

 loaded. The wrist joint is fixed just in the same way as 

 the joints of the foot are made rigid by muscles when it 

 has to serve as a lever. Even when we take a pen in our 

 hand and write, these engines which balance and fix the 

 wrist have to be in action all the time. The steadiness of 

 our writing depends on how delicately they are balanced. 

 Like the muscles of the foot, the fixers of the wrist may 

 become overworked and exhausted, as occasionally happens 

 in men and women who do not hold their pens correctly 

 and write for long spells day after day. The break-down 

 which happens in them is called " writer's cramp," but it is 

 a disaster of the same kind as that which overtakes the foot 

 when its arch collapses, and its utility as a lever is lost. 



