BONES AND JOINTS OF THE EXTREMITIES 55 



play in its socket, many hundred times in an 

 hour, for sixty years together, without diminution 

 of its agility, which is a long time for anything to 

 last, for anything so much worked and exercised 

 as the joints are. This durability I should attrib- 

 ute, in part, to the provision which is made for 

 the preventing of wear and tear : first, by the 

 polish of cartilaginous surfaces; secondly, by the 

 healing lubrication of the mucilage ; and, in part, 

 to that astonishing property of animal constitu- 

 tions, assimilation, by which, in every portion of 

 the body, let it consist of what it will, substance 

 is restored and waste repaired." PALEY. 



If the ingenious author's mind had been pro- 

 fessionally called to contemplate this subject, he 

 would have found another explanation. There is 

 no resemblance betwixt the provisions against the 

 wear and tear of machinery and those for the 

 preservation of a living part. As the structure 

 of the parts is originally perfected by the action 

 of the vessels, the function or operation of the 

 part is made the stimulus to those vessels. The 

 cuticle on the hands wears away like a glove ; but 

 the pressure stimulates the living surface to force 

 successive layers of skin under that which is wear- 

 ing, or, as the Anatomists call it, desquamating ; 

 by which they mean, that the cuticle does not 

 change at once, but comes off in squamce, or 

 scales. The teeth are subject to pressure in chew- 



