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the head of the thigh-bone is inferted, is deeper, and made of more folid materials. This agrees with the duties affigned to each part. The arm is an inftrument of motion, principally, if not folely. Accordingly the fhallowness of the focket at the fhoulder, and the yieldingness of the cartilaginous substance with which its edge is fet round, and which in fact composes a confiderable 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 fupport the body, as well as to be the means of its locomotion; firmness was to be confulted 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 ftability, or resistance to diflocation. Hence the deeper excavation of the focket; and the prefence of a lefs proportion of cartilage upon the edge.

The fuppleness and pliability of the joints we every moment experience; and the firmnefs of animal articulation, the property we have hitherto been confidering, may be judged

of

of from this fingle obfervation, 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, bloodveffels, and tendons, which are neceffary 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 fudden motions and from abrupt changes of curvature, to encounter the danger of compreffion, attrition, or laceration. To guard fibres fo tender against confequences fo 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 fupply 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

compofes

composes the upper part of the arm*. At the knee the extremity of the thigh-bone is divided by a finus or cliff into two heads or protuberances; and thefe heads on the back part fland 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 hamftrings, and within the concave recess of the bone formed by the extuberances on each side; in a word, along a defile, between rocks, pafs the great veffels and nerves which go to the legt. Who led thefe veffels by a road so defended and fecured? In the joint at the foulder, 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 bloodveffels fteal to their deftination in the arm, inftead of mounting over the edge of the concavity.

Ill. In all joints, the ends of the bones, which work against each other, are tipped with grifle. In the ball and focket joint, the cup is lined, and the ball capped with it. The

Chef. An. p. 255, ed. 7th.

+ Ib. p. 35.

Ib. 30. fmooth

fmooth furface, the elaftic and unfriable nature of cartilage, render it of all fubftances. the propereft for the place and purpose. I fhould therefore have pointed this out amongst the foremost of the provifions which have been made in the joints for the facilitating of their action, had it not been alledged, that cartilage in truth is only nafcent or imperfect bone; and that the bone in these places is kept foft and imperfect, in confequence of a more complete and rigid offification being prevented from taking place by the continual motion. and rubbing of the furfaces. Which being fo, 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 fo, it anfwers the argument. To me, the furmounting of the ends of the bones with griftle, looks more like a plating with a different metal, than like the fame metal kept in a different ftate by the action to which it is expofed. At all events we have a great particular benefit, though arifi g from a general conftitution: but this laft not being quite what my argument requires, left I fhould feem by applying the intance, to overrate its value, I have thought

it fair to state the queftion which attends it.

IV. In fome joints, very particularly in the knees, there are loofe cartilages or griftles between the bones, and within the joint, fo that the ends of the bones, inftead of working upon one another, work upon the intermediate cartilages. Chefelden has observed *, That the contrivance of a loose ring is practifed by mechanics, where the friction of the joints of any of their machines is great; as between the parts of crock 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 feen, which a mechanic reforts to, only when fome ftrong 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 preffing upon it, and often, as in rifing from our feat, with the whole weight of the body

Ib. p. 13.

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