92 MECHANISM OF THE BONES. 



the limbs at the knee to a most awkward and uncom- 

 fortable angle. Again, had the ball and socket been 

 placed at the knee, it would have been less firm than 

 the hinge joint ; " and there would have been no 

 use that we know of, in being able to turn the calves 

 of our legs before." 



The bones are all cased over at the joints with firm, 

 leathern-like caps, resembling the rings which artists 

 employ to prevent the joints of their machinery from 

 wearing away. The substance is gristle, and it is 

 found in no other part of the bones, but at the joints. 

 Besides this, there is around every joint a little bag 

 which passes from bone to bone, containing a liquid, 

 anatomists call it the synovia, which keeps the surface 

 smooth, and is exactly the same, in effect, with the 

 oil which mechanics employ for a similar purpose. 

 The joints, in short, are both leathered and oiled. 



This fluid is vulgarly called joint-oil, but it has no 

 property of oil. It is more like mucilage, smooth and 

 slippery to the touch ; and therefore better adapted, 

 than any oil to lubricate the interior of the joints and 

 prevent ill effects from friction. It is regularly sup- 

 plied by means of a membrane in the joint furnished 

 with little glands, which pour it out as it is wanted. 

 " A late improvement,' observes Dr. Paley, * in what 

 are called friction wheels, which consists of a mechan- 

 ism so ordered, as to be regularly dropping oil into a 

 box, which encloses the axis, the nave, and certain 

 balls upon which the nave revolves, may be said, in 

 some sort, to represent the contrivance in the animal 

 joint ; with this superiority, however, on the part of 



