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animal mechanism, with which we are acquainted. The chain of a watch, (I mean the chain which passes between the spring-barrel and the fusee) which aims at the fame properties, is but a bungling piece of workmanship in comparison with that of which we speak.
IV. The reciprocal enlargement and contraction of the chest to allow for the play of the lungs, depends upon a simple yet beautiful mechanical contrivance, referable to the structure of the bones which inclose it. The ribs are articulated to the back bone, or rather to its side projections, obliquely; that is, in their natural position they bend or slope from the place of articulation downwards. But the basis upon which they rest at this end being fived, the consequence of the obliquity, or the inclination downwards, is, that, when they come to move, whatever pulls the ribs upwards, necessarily, at the fame time, draws them out; and that, whilst the ribs are brought to a right angle with the ipine behind, the sternum, or part of the chest to which they are attached in front, is thrust forward. The si" pve action, there fore, of the elevating missel s does the business; whereas, if the ribs had been articulated with the bodies of the vertebræ tebræ at right angles; the cavity of the thorax cc uld never have been further enlarged by a change of their position. If each rib had bee a' a rigid bone, articulated at both ends to fixed bas s, the whole chest had been immovable. Kcill has observed, that the breast-bone, in an easy in pir.ition, is thrust out one tenth of an inch; and he calculates that this, added to' what is gained to the space within the chest by the flattening or descent of the diaphragm, leaves room for forty-two cubic inches of air to enter at every drawing in of the breath. When there is a necessity for a deeper and more laborious inspiration, the enlargement of the capacity of the chest nr.y be so increased by effort, as that the lungs may be distended with seventy or a hundred such cubic inches*. The thorax, fays Schelhammer, forms a kind of bellows, such as never have been, nor probably will be, made by any artificer.
V. The patella, or knee-pan, is a curious little bone; in its form and office unlike any other bone of the body. It is circular; the size of a crown piece; pretty thick; a little convex on both sides, and covered with a
* Annt. p. 229. - j
1 2 smooth smooth cartilage. It lies upon the front of the knees 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 suffer, by the rubbing of one against the other, or by the pressure of unequal surfaces. It also gives to the tendons a very considerable 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 resolution of force, upon which principles all machinery is founded. These are its uses. But what is most observable in it is, that it appears to be supplemental, as it were, to the frame; added, as it should almost seem, afterward ; not quite necessary, but very convenient. It is separate from the other bones; that is, it is not connected with any other bones by the common mode of union. It is soft, or hardly formed, in infancy; and produced by an ossification, of the inception or progress of which, no account can be given from the structure or exercise of the part.
VI. The JJjouider-blade is, in some material respects, a very singular bone: it appearing to be made so expressly for its own purpose, and so independently of every other reason. In such quadrupeds as have no collar-bones, which are by far the greater number, the shoulder-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 second bone in the body, except perhaps the os hyoides). In strictness, 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 ossification. 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 balls, whereupon the arm might be articulated, was to be supplied by a detached ossification 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 iurm or stiape.of the bones themselves, are teen both con'rnance and contriving wildrm. Evety joint is a curiosity, and is also strict y mechanical, There is the hinge j "in , and the mortice and tenon .pint; each as manifestly such, and as accurately defined, as any which can be produced out of a cabinet-maker's sh op. 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 atthe knee,the leg stanJingin need only of a motion backward and forw ird in the fame plane, for which a hinge joint is sufficient: a mortice and tenon, or ball and socket joint, is wanted at the hip, that not only the progressive flip may he provided for, but the interval between the limbs may be enlarged or contracted at pleasure. Now observe what would have been the inconveniency, i. e. both the superfluity and the e'esect of articulation, if the case had been inverted; if the b ill and socket joint had been at the knee, and the hinge joint at the hip. The thighs must have been kept constantly together, and the legs have been loose and straddling, There would have been