72 OF MECHANICAL ARRANGEMENT 



tween 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 reason why 

 we find them in the knees rather than in other joints. It 

 is an expedient, we have seen, which a mechanic 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- 

 selden's account, that the slipping and sliding of the loose 

 cartilages, though it be probably a small and obscure 

 change, humoured 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, and that 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 linament, 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 

 port of the joints, viz. that here, the oil is not only dropped, 

 but made, t 



* 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. 



Farton, 



t A joint then consists of the union of two bon^s, of such a form as to 

 permit the necessary motion ; but they are not in contact j each artic- 



