262 CURVATURES AND MOVEMENTS OF THE 



35. It appears necessary, however, at this point to anticipate 

 so far by recording a few observations on the teleological re- 

 lations or final purposes of these peculiar principles of con- 

 struction which characterise organic joints. 



From the peculiar character of the curvature which obtains 

 in organic joints, all points of their opposite surfaces come 

 successively into and then break contact, so that these joints 

 perform a maximum of work with a minimum of surface- 

 contact ; while, at the same time, that weakness which is en- 

 tailed by diminution of surface-contact on the organic as well 

 as on artificial joints, is compensated for by muscular action 

 in the former during its movements to and from its negative 

 position, whereas the latter must in all its positions support 

 itself. Both the organic and the artificial joints derive ad- 

 vantage from diminished surface-contact, for their deterioration 

 is lessened by the diminished amount of pressure and friction. 

 But the deterioration of the materials and structure of an 

 organic joint during action may be assumed as proportionally 

 greater than that of an artificial joint, for in addition to the 

 actual injury to structure sustained, there is, during action, a 

 suspension more or less complete of those nutritive processes 

 on which the ulterior integrity of the organic joint surface 

 depends. The artificial joint surfaces consist of materials 

 which may be assumed as not subject to deterioration of their 

 molecular constitution, but only in their configuration during 

 action from pressure and attrition. This deterioration cannot 

 be remedied by the economy of the joint, but accumulates 

 with duration of action, and ultimately demands the interfer- 

 ence of the mechanician. The organic joint surfaces again 

 not only suffer from pressure and attrition, but in their 

 molecular constitution as well, and as this molecular deterio- 

 ration diminishes the efficiency of their ulterior action, the 

 organic joint is subjected to more speedy injury in the per- 

 formance of its function than the artificial. 



