fmooth cartilage. It lies upon the front of the knee; and the powerful tendons, by which the leg is brought forward, pass through it (or rather it makes a part of their continuation) from their origin in the thigh to their insertion in the tibia. It protects both the tendon and the joint from any injury which either might fuffer, by the rubbing of one against the other, or by the preffure of unequal furfaces. It alfo gives to the tendons a very confiderable mechanical advantage by altering the line of their direction, and by advancing it further out from the centre of motion; and this upon the principles of the refolution of force, upon which principles all machinery is founded. These are its ufcs. But what is moft obfervable in it is, that it appears to be fupplemental, as it were, to the frame; added, as it should almoft feem, afterward; not quite neceffary, but very convenient. It is feparate from the other bones; that is, it is not connected with any other bones by the common mode of union. It is foft, or hardly formed, in infancy; and produced by an offification, of the inception or progrefs of which, no account can be given from the structure or exercife of the part.

VI. The

VI. The boulder-blade is, in fome material respects, a very fingular bone: it appearing to be made fo exprefsly for its own purpose, and so independently of every other reason. In fuch quadrupeds as have no collar-bones, which are by far the greater number, the fhoulder-blade has no bony communication with the trunk, either by a joint, or process, or in any other way. It does not grow to, or out of, any other bone of the trunk. It does not apply to any other bone of the trunk (I know not whether this be true of any fecond bone in the body, except perhaps the os hyoides). In ftrictness, it forms no part of the skeleton. It is bedded in the flesh; attached only to the muscles. It is no other than a foundation bone for the arm, laid in, separate, as it were, and distinct, from the general offification. The lower limbs connect themselves at the hip with bones which form part of the skeleton; but, this connection, in the upper limbs, being wanting, a basis, where upon the arm might be articulated, was to be fupplied by a detached offification for the purpose.

I. THE ABOVE are a few examples of bones made remarkable by their configuration: but

to almost all the bones belong joints; and in these, still more clearly than in the form or fhape of the bones themselves, are seen both contrivance and contriving wifdom. Every joint is a curiofity, and is alfo ftrictly mechanical. There is the hinge joint, and the mortice and tenon joint; each as manifeftly fuch, and as accurately defined, as any which can be produced out of a cabinet-maker's shop. And one or the other prevails, as either is adapted to the motion which is wanted: e. g. a mortice and tenon, or ball and socket joint, is not required at the knee, the leg standing in need only of a motion backward and forward in the fame plane, for which a hinge joint is fufficient: a mortice and tenon, or ball and focket joint, is wanted at the hip, that not only the progreffive ftep may be provided for, but the interval between the limbs may be enlarged or contracted at pleasure. Now obferve what would have been the inconveniency, i. e. both the superfluity and the defect of articulation, if the case had been inverted; if the ball and focket joint had been at the knee, and the hinge joint at the hip. The thighs muft have been kept conftantly together, and the legs have been loose and straddling. There would have been

no

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 lefs, if the joints at the hip and the knee had been both of the fame fort; both balls and fockets, or both hinges: yet why, independently of utility, and of a Creator who confulted that utility, should the fame bone (the thigh-bone) be rounded at one end, and channelled at the other?

The hinge joint is not formed by a bolt paffing through the two parts of the hinge, and thus keeping them in their places; but by a different expedient. A ftrong, tough, parchment-like membrane, rifing from the receiving bones, and inferted all round the received bones a little below their heads, inclofes the joint on every fide. 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 focket joint, beside the membrane already defcribed, there is in fome important joints, as an additional security, a short,

I 4

fhort, ftrong, yet flexible ligament, inferted, 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 fo firmly in their place, that none of the motions which the limb naturally performs, none of the jerks and twifts to which it is ordinarily liable, nothing lefs indeed than the utmoft and the most unnatural violence, can pull them afunder. It is hardly indeed imaginable, how great a force is neceffary, even to stretch, ftill more to break, this ligament; yet fo flexible is it, as to oppofe no impediment to the fuppleness of the joint. By its fituation. alfo, it is inacceffible to injury from fharp edges. As it cannot be ruptured, fuch is its ftrength; fo it cannot be cut, except by an accident which would fever the limb. If I had been permitted to frame a proof of contrivance, fuch as might fatisfy the most diftruftful enquirer, I know not whether I could have chofen an example of mechanism more unequivocal, or more free from objection, than this ligament. Nothing can be more mechanical; nothing, however fubfervient to the fafety, lefs capable of being generated

by