72 
OF MECHANICAL ARRANGEMENT 
contrivance of a loose ring is practised by mechanics, where 
the friction of the joints of any of their machines is great; 
as between the parts of crooked-hinges of large gates, or 
under the head of the male screw of large vices. The 
cartilages of which we speak, have very much of the form 
of these rings. The comparison moreover shows the rea¬ 
son why we find them in the knees rather than in other 
joints. It is an expedient, we have seen, which a mechan¬ 
ic resorts to, only when some strong and heavy work is to 
be done. So here the thigh bone has to achieve its motion 
at the knee, with the whole weight of the body pressing 
upon it, and often, as in rising from our seat, with the whole 
weight of the body to lift. It should seem also, from Ches- 
elden’s account, that the slipping and sliding of the loose 
cartilages, though it be probably a small and obscure 
change, humored the motion of the end of the thigh-bone, 
under the particular configuration which was necessary to 
be given to it for the commodious action of the tendons; 
(and which configuration requires what he calls a variable 
socket, that is, a concavity, the lines of which assume a 
different curvature in different inclinations of the bones.) 
V. We have now done with the configuration: but 
there is also in the joints, andi;hat common to them all, 
another exquisite provision, manifestly adapted to their use, 
and concerning which there can, I think, be no dispute, 
namely, the regular supply of a mucilage, more emollient 
and slippery than oil itself, which is constantly softening 
and lubricating the parts that rub upon each other, and 
thereby diminishing the effect of attrition in the highest 
possible degree.* For the continual secretion of this im¬ 
portant liniment, and for the feeding of the cavities of the 
joint with it, glands are fixed near each joint; the excre¬ 
tory ducts of which glands dripping with their balsamic 
contents, hang loose like fringes within the cavity of the 
joints. A late improvement in what are called friction 
wheels, which consists 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 revolves, 
may be said, in some sort, to represent the contrivance in 
the animal joint; with this superiority, however, on the 
* This mucilage is termed synovia ; vulgarly called joint oil, but it 
has no property of oil. It is very viscid, and at the same time 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. 
Paxton 
