86 THE ENGINES OF THE HUMAN BODY 



to bear on these surfaces, the greater is the response of 

 the cartilage-builders, the more rapid is their destruction, 

 and therefore the more plentiful the supply of synovia. 

 But what becomes of the waste lubricant — for even a 

 film of oil is not permanent ? Every lubricant undergoes 

 changes which, in course of time, unfit it for further 

 service, hence it must be drained ofF and replaced. All of 

 Nature's joints are completely enclosed spaces — enclosed 

 by flaccid membranes which are loose enough to allow the 

 adjacent levers to carry out all their normal movements 

 (fig. 23). There is no empty space in Nature's joint 

 boxes ; synovial covered surface rubs on the surface set 

 against it ; only a lubricant film of synovia lies between 

 them. There is no drain-pipe for the escape of lubricant. 

 Nature is too economical for that. Into the crevices of 

 the joints there project certain soft, warty fringes which 

 have the power of absorbing and returning to the general 

 system by way of the blood stream all the waste lubricant 

 produced at articular surfaces. 



Many men and women pass through life and never 

 have cause to know that the easy movement of the 

 human machine is dependent on a lubricant system — 

 so perfectly does it serve their needs. Even at the 

 age of seventy human axles and bearings may be as 

 fresh and unworn as in the hey-day of youth. That 

 bugbear of the motor-driver — the sudden " seizing " of 

 joints — never troubles the drivers of the human machine. 

 And yet to many people — especially as years crowd over 

 them — a peculiar kind of " seizing " does happen. From 

 some cause we have not yet discovered, the lubricant 

 system fails — in one joint or in several. Then the 

 one cartilage plate begins to rub stiffly on its opponent, 

 there is friction, and the articular plate wears* in patches. 

 The cartilage-builders, in place of melting down and 

 becoming a slippery lubricant, remain obdurate and 

 fibrous. The cartilage plates become leathery in con- 

 sistence and ultimately worn through in places. The 

 neighbouring bone-builders, which are saved from all 

 forms of friction in healthy joints, become disturbed by 



