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animal mechanifm, with which we are acquainted. The chain of a watch, (I mean the chain which paffes between the spring-barrel and the fufec) which aims at the fame properties, is but a bungling piece of workmanship in comparison with that of which we speak.

The

IV. The reciprocal enlargement and contraction of the cheft to allow for the play of the lungs, depends upon a fimple yet beautiful mechanical contrivance, referable to the ftructure of the bones which inclofe it. ribs are articulated to the back bone, or rather to its fide projections, obliquely; that is, in their natural pofition they bend or flope from the place of articulation downwards. But the bafis upon which they reft at this end being fived, the confequence of the obliquity, or the inclination downwards, is, that, when they come to move, whatever pulls the ribs upwards, neceffarily, at the fame time, draws them out; and that, whilft the ribs are brought to a right angle with the fpine behind, the fternum, or part of the cheft to which they are attached in front, is thraft forward.

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fin p'e action, there fore, of the elevating musels dos the bu'inels; whereas, if the ribs had been articulated with the bodies of the ver

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tebræ at right angles; the cavity of the thorax could never have been further enlarged by a change of their position. If each rib had been a rigid bone, articulated at both ends to fixed. bafes, the whole cheft had been immovable. Keill has obferved, that the breaft-bone, in an eafy in piration, is thrust out one tenth of an inch; and he calculates that this, added to' what is gained to the fpace within the cheft by the flattening or defcent 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 neceflity for a deeper and more laborious infpiration, the enlargement of the capacity of the cheft may be fo increased by effort, as that the lungs may be diftended with feventy or a hundred fuch cubic inches*. The thorax, fays Schelhammer, forms a kind of bellows, fuch 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 fize of a crown piece; pretty thick; a little convex on both fides, and covered with a

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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 infertion 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 also 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 resolution of force, upon which principles all machinery is founded. These are its ufes. But what is moft obfervable in it is, that it appears to be fupplemental, as it were, to the frame; added, as it should aloft 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 progress of which, no account can be given from the ftructure 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 fo 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 mufcles. It is no other than a foundation bone for the arm, laid in, separate, as it were, and diftinct, from the general offification. The lower limbs connect the mselves at the hip with bones which form part of the skeleton; but, this connection, in the upper limbs, being wanting, a basis, whereupon 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

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to almost all the bones belong joints; and in thefe, ftill more clearly than in the form or fhape of the bones th. mfelves, are feen both contrivance and contriving wildem. Ever joint is a curiofity, and is alfo ftrict y nec anical. There is the hinge join, 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 fh 'P. 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 focket 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 ftp 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 fuperfluity and the defect of articulation, if the cafe had been inverted; if the ball and focket joint had been at the knee, and the hinge joint at the hip. The thighs must have been kept conftantly together, and the legs have been loose and straddling, There would have been

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