NATURAL THEOLOGY. 129 



diminishing the effect of attrition in the highest pos^ 

 sible degree. For the continual secretion of this 

 important liniment, and for the feeding of the cavi- 

 ties of the joint with it, glands are fixed near each 

 joint, the excretory ducts of which glands, dripping 

 with their balsamic contents, hang loose like fringes 

 within the cavity of the joints. A late improve- 

 ment in what are called friction wiieels, which con- 

 sist of a mechanism so ordered as to be regularly 

 dropping oil into a box which encloses the axis, the 

 nave, and certain balls upon which the nave re- 

 volves, may be said, in some sort, to represent the 

 contrivance in the animal joint, with this superior- 

 ity, however, on the part of the joint, viz., that here 

 the oil is not only dropped, but made. 



In considering the joints, there is nothing, per- 

 haps, which ought to move our gratitude more than 

 the reflection, how well they wear. A limb shall 

 swing upon its hinge, or 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 dura- 

 bility I should attribute in part to the provision which 

 is made for the preventing of wear and tear, first by 

 the polish of the cartilaginous surfaces ; secondly, 

 by the healing lubrication of the mucilage, and, in 

 part, to that astonishing property of animal consti- 

 tutions, assimilation, by which, in every portion of 



