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the head of the thigh-bone is inserted, is deeper, and made of more solid materials. This agrees with the duties assigned to each part. The arm is an instrument of motion, principally, if not solely. Accordingly the shallowness of the socket at the shoulder, and the yieldingness of the cartilaginous substance with which its edge is set round, and which in fact composes a considerable part of its concavity, are excellently adapted for the allowance of a freer motion and a wider range; both which the arm wants. Whereas the lower limb, forming a part of the column of the body ; having to support the body, as well as to be the means of its locomotion ; firmness was to be consulted as well as action. With a capacity for motion, in all directions indeed, as at the shoulder, but not in any direction to the fame extent as in the arm, was to be united stability, or resistance to dislocation. Hence the deeper excavation of the socket; and the presence of a less proportion of cartilage upon the edge.
The suppleness and pliability of the joints we every moment experience; and the jinnness of animal articulation, the property we have hitherto been considering, may be judged
of from this single observation, that, at any given moment of time, there are millions of animal joints in complete repair and use, for one that is dislocated; and this notwithstanding the contortions and wrenches to which the limbs of animals are continually subject.
II. The joints, or rather the ends of the bones which form them, display also, in their ^configuration, another use. The nerves, bloodvessels, and tendons, which are necessary to the life, or for the motion, of the limbs, must, it is evident, in their way from the trunk of the body to the place of their destination, travel over the moveable joints; and it is no less evident, that, in this part of their course, they will have, from sudden motions and from abrupt changes of curvature, to encounter the danger of compression, attrition, or laceration. To guard fibres so tender against consequences so injurious, their path is in those parts protected with peculiar care: and that by a provision in the figure of the bones themselves. The nerves which supply the fore arm, especially the inferior cubital nerves, are at the elbow conducted, by a kind of covered way, between the condyls, or rather under the inner extuberances of the bone, which
composes composes the upper part of the arm*, At the knee (he extremity of the thjgi-bone is divided by a sinus cr cliff into two heads or protuberances; and these fields on the back part stand out beyond the cylinder of the bone. Through the hollow, which lies between the hind parts of these two heads, that is to fay, under the ham, between the hamstrings, and within the concave recess of We bone formed by the extuberances on each side; in a word, along a defile, between rocks, pass the great vessels and nerves which go to the legs. Who led t! ese vessels by a road so defended and secured? In the joint at the Jhoulder, in the edge of the cup which receives the head of the bone, is a notch which is joined or covered at the top with a ligament. Through this hole, thus guarded, the bloodvessels steal to their destination in the arm, in (had of mounting over the edge of theconcavLy X'
III. In all joints, the ends of the bones, which work against each other, are tipped with gri/IIe. In the ball and socket joint, the cup is lined, and the ball capped with it. The
• Chef. An. p. 255, ed. 7th. fib. p. 35. \ Ib. 30.
smooth surface, the elastic and unfriable nature of cartilage, render it of all substances the properest for the place and purpose. I would therefore have pointed this out amongst the foremost of the provisions which have been made in the joints for the facilitating of their action, had it not been alledged, that cartilage in truth is only nascent or impersect bone ; and that the bone in these places is kept soft and impersect, in consequence of a more complete and rigid ossification being prevented from taking place by the continual motion and rubbing of the surfaces. Which being so, what we represent as a designed advantage, is an unavoidable effect. I am far from being convinced that this is a true account of the fact; or that, if it were so, it answers the argument. To me, the surmounting of the ends of the bones with gristle, looks more like a plating with a different metal, than like the fame metal kept in a different state by the action to which it is exposed. At all events we have a great particular benefit, though arid, g from a general constitution: but this last not being quite what my argument requires, lest I should seem by applying the instance, to overrate its value, I have thought
it fair to state the question which attends
IV. In some joints, very particularly in the knees, there are loose cartilages or gristles between the bones, and within the joint, so that the ends of the bones, instead of working upon one another, work upon the intermediate cartilages. Cheselden has observed *, That the 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 crook 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 shews 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 thighbone 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
* Ib. p. 13.