66 NATUHAL THEOLOaY. 



dripping with their balsamic contents, hang loose like fringes 

 v/ithin the cavity of the joints. A late improvement in 

 what are called friction v/heels, 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 rep- 

 resent the contrivance in the animal joint, with this superi- 

 ority, however, on the part of the joint, namely, that here 

 the oil is not only dropped, but made. 



In considering the joints, there is nothing, perhaps, which 

 ought to move our gratitude more than the reflection, how 

 icell theij ivear. 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 any thing to last — for any thing so much worked and 

 exercised as the joints are. This durability I should attribute 

 m part to the provision which is made for the preventing of 

 wear and tear, first by the polish of the cartilaginous surfac- 

 es ; secondly, by the healing lubrication of the mucilage, and 

 in part, to that astonishing property of animal constitutions, 

 assimilation, by which, in every portion of the body, let it con- 

 sist of what it will, substance is restored and waste repaired. 



Movable joints, I think, compose the curiosity of bones ; 

 but their union, even where no motion is intended or want- 

 ed, carries marks of mechanism and of mechanical wisdom. 

 The teeth, especially the front teeth, are one bone fixed in 

 another, like a peg driven into a board. The sutures of the 

 Bkull=^ are like the edges of two saw^s clapped together in such 

 a manner as that the teeth of one enter the intervals of the 

 oth?r. We have sometimes one bone lapping over another, 

 and planed down at the edges ; sometimes also the thin lamel- 

 la of one bone received into a narrow furrow of another. In 

 all which varieties we seem to discover the same design, 

 namely, firmness of juncture without clumsiness in the seam. 



* Plate XL, Fig. 6. a, a, the coronal suture ; b, the sagittal ; 

 c. c, tJie lamlidoidal ; d, an irregularity ; and e, e, the squamous sutures- 



