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assignable direction. “I think,” says Cheselden, “that the knee cannot be completely dislocated without breaking the cross ligaments".” We can hardly help comparing this with the binding up of a fracture, where the fillet is almost always strapped across, for the sake of giving firmness and strength to the bandage. Another no less important joint, and that also of the ginglymus sort, is the ankle; yet though important (in order, perhaps, to preserve the symmetry and lightness of the limb), small, and, on that account, more liable to injury. Now this joint is strengthened, i. e. is defended from dislocation, by two remarkable processes or prolongations of the bones of the leg: which processes form the protuberances that we call the inner and outer ankle. It is part of each bone going down lower than the other part, and thereby overlapping the joint: so that, if the joint be in danger of slipping outward, it is curbed by the inner projection, i. e. that of the tibia; if inward, by the outer projection, i. e. that of the fibula. Between both, it is locked in its position. I know no account that can be given of this structure, except its utility. Why should the tibia terminate, at its lower * Ches, Anat. ed. 7th, p. 45.
extremity, with a double end, and the fibula the same, but to barricade the joint on both sides by a continuation of part of the thickness of the bone over it? The joint at the shoulder compared with the joint at the hip, though both ball and socket joints, discovers a difference in their form and proportions, well suited to the different offices which the limbs have to execute. The cup or socket at the shoulder is much shallower and flatter than it is at the hip, and is also in part formed of cartilage set round the rim of the cup. The socket, into which 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 free motion and a wide 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 same 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 firmmess 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, blood-vessels, 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 the upper part of the arm”. At the knee, the extremity of the thigh-bone is divided by a sinus or cliff into two heads or protuberances: and these heads 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 say, under the ham, between the ham-strings, and within the concave recess of the 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 legi. Who led these vessels by a road so defended and secured? In the joint at the shoulder, 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 blood-vessels steal to * Ches. Anat, p. 255, ed. 7. + Ib. p. 35.
their destination in the arm, instead of mounting over the edge of the concavity*. III. In all joints, the ends of the bones, which work against each other, are tipped with gristle. In the ball and socket joint, the cup is lined, and the ball capped with it. The smooth surface, the elastic and unfriable nature of cartilage, render it of all substances the most proper for the place and purpose. I should, 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 alleged, that cartilage in truth is only nascent or imperfect bone; and that the bone in these places is kept soft and imperfect, in consequence of a maore 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 same metal kept in a different state by the action to which it Ches. Anat. ed. 7, p. 30.