IN THE HUMAN FRAME. 73 



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

 ought to move our gratitude more than the reflection, lioio 

 well they wear. A limb shall swing upon its hinge, or 

 play in its socket, many hundred times in an hour, for six- 

 ty 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 durabiUty 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 constitutions, assimilation, by which, in every 

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

 is restored, and waste repaired. * 



ulating surface is covered with cartilage, to prevent the jar which 

 would result from the contact of the bones. This cartilage is elastic, 

 and the celebrated Dr. Hunter discovered that the elasticity was ia 

 consequence of a number of filaments closely compacted, and extend- 

 ing from the surface of the hone, so that each filament is perpendicu- 

 lar to the pressure made upon it. The surface of the articulating car- 

 tilage is perfectly smooth, and is lubricated by a fluid called synovia, 

 signifying a mucilage, a viscous or thick liquor. This is vulgarly 

 called joint oil, but it has no property of oil, although it ia better cal- 

 culated than any oil to lubricate the interior of die joint 



When inflammation comes upon a joint, this fluid is not supplied, and 

 the joint is stiff, and the surfaces creak upon one another Uke a hinge 

 without oil. A delicate membrane extends from bone to bone, confin- 

 ing this lubricating fluid, and forming the boundary of what is termed 

 the cavity of the joint, although, in fact, there is no unoccupied space. 

 External to this capsule of the joint, there are strong ligaments going 

 from point to point of the bones, and so ordered as to bind them to- 

 gether without preventing their proper motions. From this description 

 of a single joint, we can easily conceive what a spring or elasticity is 

 given to the foot, where thirty-six bones are joined together. 



BeWs Treatise on Animal Mechanics. 



* If the ingenious author's mind had been professionally 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 

 tliose vessels. The cuticle on the hands wears away like a glove ; 

 but the pressure stimulates tlie living surface to force successive lay- 

 ers of skin under that which is wearing, 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 chewing or masticating, and they would, by this action, have 

 been driven deeper in the jaw, and rendered useless, had there not 

 been a provision against this mechanical effect. This provision is a 

 disposition to grow, or rather to shoot out of their sockets ; and this dis- 

 position to project, balances the pressure which they sustain ; and 

 when one tooth is lost, its opposite rises, and is in danger of being lost 

 also, for want of that very opposition. Ibid^ 



