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no use that we know of, in being able to turn the calves of the legs before ; and there would have been great confinement by restraining the motion of the thighs to one plane. The disadvantage would not have been less, if the joints at the hip and the knee had been both of the fame fort; both balls and sockets, or both hinges : yet why, independently of utility, and of a Creator who consulted that utility, should the fame bone (the thigh-bone) be rounded at one end, andchannelledat the other?
The binge joint is not formed by a bolt passing through the two parts of the hinge, and thus keeping them in their places; but by a different expedient. A strong, tough, parchment-like membrane, rising from the receiving bones, and inserted all round the received bones a little below their heads, incloses the joint on every side. This membrane ties, confines, and holds the ends of the bones together; keeping the corresponding parts of the joint, i. e. the relative convexities and concavities, in close application to each other.
For the ball and socket joints beside the membrane already described, there is in some important joints, as an additional security, a
I 4 short, short, strong, yet flexible ligament, inserted, by one end into the head of the ball, by the other into the bottom of the cup; which ligament keeps the two parts of the joint so firmly in their place, that none of the motions which the limb naturally performs, none of the jerks and twists to which it is ordinarily liable, nothing less indeed than the utmost and the most unnatural violence, can pull them asunder. It is hardly indeed imaginable, how great a force is necessary, even to stretch, still more to break, this ligament; yet so flexible is it, as to oppose no impediment to the suppleness of the joint. By its situation also, it is inaccessible to injury from sharp edges. As it cannot be ruptured, such is its strength; so it cannot be cut, except by an accident which would sever the limb. If I had been permitted to frame a proof of contrivance, such as might fatisfy the most distrustful enquirer, I know not whether I could have chosen an example of mechanism more unequivocal, or more free from objection, than this ligament. Nothing can be more mechanical; nothing, however subservient to the fasety, less capable of being generated by the action of the joint. I would particularly solicit the reader's attention to this provision, as it is found in the head of the thigh-bone; to its strength, its structure, and its use. It is an instance upon which I lay my hand. One single fact, weighed by a mind in earnest, leaves oftentimes the deepest impression. For the purpose of addressing different understandings and different apprehensions, for the purpose of sentiment, for the purpose of exciting admiration of the Creator's works, we diversify our views, we multiply examples; but, for the purpose of strict argument, one clear instance is sufficient: and not only sufficient, but capable perhaps of generating a firmer assurance than what can arise from a divided attention.
The ginglymus, or hinge joint, does not, it is manifest, admit of a ligament of the fame kind with that of the ball and socket joint, but it is always fortified by the species of ligament of which it does admit. The strong, firm, investing membrane above described, accompanies it in every part: and, in particular joints, this membrane, which is properly a ligament, is considerably stronger Qn the sides than either before or behind, in 3 t order order that the convexities may play true in their concavities, and not be subject to slip sideways, which is the chief danger; for the muscular tendons generally restrain the parts from going further than they ought to go in the plane of their motion. In the knee, which is a joint of this form, and of great importance, there are fuperadded to the common provisions for the stability of the joint, two strong ligaments which cross each other; and cross each other in such a manner, as to secure the joint from being displaced in any assignable direction. "I think," fays Chesclden, " that the knee cannot be completely dislocated without breaking thecross ligaments *." We can hardly help comparing this with the binding up of a fracture, where the fillet is almost always strapped across, for the fake 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.
* Chef. Anat. ed. 7th, p. 45.
is is defended f.om 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 j >int: so that, if the joint be in danger of flipping outward, it is curbed by the inner projection, i. e. that of the tibia; if inward, by the outer production, i. e. that of the fibula. Between both, it is locked in its position. I know no account that can be given of tnis structure except its utility. Why should the tibia terminate, at its lower 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, discover a difference in their form and proportions, well suited to the difserent offices which the limbs have to execute. The cup or socket at the shoulder is mu h 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