ACTING FACETS OF ARTICULAR SURFACES. 263 



But this liability of the acting surfaces of an organic joint 

 to molecular deterioration during action, while it would sub- 

 ject the joint to greater injury from a given amount of work 

 than an artificial joint, is compensated for by the curvature of 

 its articular surfaces, which provides for that amount of time 

 necessary for successive restorations in the intervals of suc- 

 cessive contacts. For to whatever extent the opposite surfaces 

 of an artificial joint may be diminished, there must always 

 remain a certain extent of its opposite surfaces in contact ; and 

 from the circular form of the transverse section, the successive 

 amounts of contact, corresponding to a series of equal changes 

 in angular position during rotation, must be in arithmetical 

 progression. But in the organic joint the successive amounts 

 of contact, corresponding to successive equal increments of 

 angular position, will be in geometrical progression, during 

 which the work done will be inversely as the intervals of 

 time in which each portion of it is performed. 



If we assume, therefore, that the deteriorating effects of 

 friction are diminished in both forms of joints by an increase 

 of the rapidity of movement during a given amount of work, 

 the advantage which is gained by the organic joint in time 

 saved is greater than that gained by the artificial joint. For 

 time is an essential condition in the restoration of organised 

 matter, deteriorated by functional action. This condition of 

 time determines that alternation or rhythm which characterises 

 all the phenomena of organised bodies. The alternate inter- 

 vals of action and inaction, of fatigue and repose, of deteriora- 

 tion and renovation, of waking and sleep, of life and death, 

 are not solely due to the recurrent cosmical conditions under 

 which organisation subsists, but essentially depend on the 

 organic character of organised matter itself. Every portion 

 of organised mechanism is constructed on principles which 

 co-ordinate it with the peculiar molecular constitution of 

 organised matter. It therefore appeared to be essential, that 



