THE BIOLOGY OF HEALTH 131 



orders of levers, and these are all illustrated iu our body 

 — the jirst order in the movement of the skull on the 

 backbone ; the second order in the foot ; the third order 

 in the forearm and hand when the elbow is the fulcrum. 

 Where one bone works on another there are cartilage- 

 building cells that make good the wear of the joint 

 surface — an impossibility in a metal engine ; and when 

 these cells have worked themselves out they are dis- 

 solved to form a lubricant. Thus our joints are kept from 

 wearing away ; thus our joints are kept supple. Like 

 the engine, the body has its lubricating arrangements. 



But the muscle-engines require to have fuel brought 

 to them, and the living levers require to be kept in good 

 order ; so we see the use of the heart as the pump of the 

 body. It drives round the body a combustion-mixture 

 (oxygen and blood-sugar) which is comparable to the 

 oxygen and petrol which is brought into the motor- 

 cycle's internal-combustion engine. In both cases 

 the waste products have to be swept away. The 

 piston of the motor-cycle is made to do the pumping 

 as well as the actual work ; in our body the pumping 

 apparatus (the heart) and the locomotor apparatus 

 (the muscles) are quite separate from one another. 



The motor-cycle would not work if it could not draw 

 in fresh air and drive out foul air; and the arrange- 

 ment for this is comparable to the pair of bellows which 

 we call our chest. Our windpipe is an air-pipe ; our 

 nose is a nozzle ; the engines that work the bellows 

 are carefully built ijito the sides aad front and floor of 

 the chest, 



