IN THE HUMAN FRAME. 
73 
part of the joint, viz. 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 
i veil 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 anything to last; for anything so much 
worked and exercised as the joints are. This durability 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.f 
* A joint then consists of the union of two bones, of such a form as to 
permit the necessary motion; but they are not in contact; each articulat¬ 
ing surface is covered with cartilage, to prevent the jar which would re¬ 
sult from the contact of the bones. This cartilage is elastic, and the 
celebrated Dr. Hunter discovered that the elasticity was in consequence 
of a number of filaments closely compacted, and extending from the sur¬ 
face of the bone, so that each filament is perpendicular to the pressure 
made upon it. The surface of the articulating cartilage is perfectly 
smooth, and is lubricated by a fluid called synovia, sygnifying a muci¬ 
lage, a viscous or thick liquor. This is vulgarly called joint oil, but it 
has no property of oil, although it is better calculated than any oil to lu¬ 
bricate the interior of the joint. 
When inflammation comes upon ajoint, this fluid is not supplied, and 
the joint is stiff, and the surfaces creak upon one another like a hinge 
without oil. A delicate membrane extends from bone to bone, confining 
this lubricating fluid, and forming the boundary of what is termed the 
cavity of the joint, although, in fact, there is no unoccupied space. Ex¬ 
ternal 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 together 
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.— Bell’s Treatise on 
Animal Mechanics. 
t If the ingenious author’s mind had been professionally called to con¬ 
template this subject, he would have found another explanation. There 
is no resemblance between the provisions against the wear and tear of 
machinery and those for the preservation of a living part. As the struc¬ 
ture 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 stim¬ 
ulates the living surface to force successive layers 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 squama, 
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 
G 
