THE HUMAN PRAME. 85 



are loose cartilages or gristles between the bones and with- 

 in the joint, so that the ends of the hones, instead of work- 

 ing upon one another, work upon the intermediate cartilages. 

 Oheselden has observed,=^ that the contrivance of a loose ring 

 is practised by mechanics where the friction of the joints oi 

 any of their machines is great, as between the parts :f ciook- 

 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 Cheselden's account, that the slipping and sliding ol 

 the loose cartilages, though it be probably a small and ob- 

 scure change, humored the motion at the end of the thigh- 

 bone, under the particular configuration which was neces- 

 sary to be given to it for the commodious action of the ten- 

 dons, and which configuration requires what he calls a vari- 

 able socket, that is, a concavity, the lines of which assume 

 a difierent curvature m different inclinations of the bones. 



y. 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 t ■. their use, and con- 

 cerning 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. Foi 

 the contmual secretion of this important liniment, and foi 

 the feeding of the cavities of the joint with it, glands are 

 fixed near each joint, the excretory ducts of which glands 

 * dies. Anat., p. 13, ed. 7. 



