to lift. It should feem alfo from Chefelden's account, that the flipping and fliding of the loose cartilages, though it be probably a small and obfcure change, humoured the motion of the end of the thigh bone, under the particular configuration which was neceffary to be given to it for the commodious action of the tendons; and which configuration requires what he calls a variable focket, that is, a concavity, the lines of which affume a different curvature in different inclinations of the bones.

V. We have now done with the configuration; but there is alfo in the joints, and that common to them all, another exquisite provifion, manifeftly adapted to their use, and concerning which there can, I think, be no difpute, namely, the regular fupply of a mucilage, more emollient and flippery than oil itself, which is conftantly foftening and lubricating the parts that rub upon each other, and thereby diminishing the effect of attrition in the highest poffible degree. For the continual fecretion of this important liniment, and for the feeding of the cavities of the joint with it, glands are fixed near each joint; the excretory ducts of which glands, dripping with their balfamic contents, hang loofe like fringes within.

the cavity of the joints. A late improvement in what are called friction wheels, which consists of a mechanism so ordered, as to be regularly dropping oil into a box, which incloses the axis, the nave, and certain balls upon which the nave revolves, may be faid in fome fort, to reprefent the contrivance in the animal joint; with this fuperiority, however, on the part of the joint, viz. that here, the oil is not only dropped, but made.

In confidering the joints, there is nothing, perhaps, which ought to move our gratitude more than the reflection, how well they wear. A limb fhall fwing upon its hinge, or play in its focket, many hundred times in an hour, for fixty years together, without diminution of its agility which is a long time for any thing to laft; for any thing fo much worked and exercifed as the joints are. This durability I fhould attribute, in part, to the provision which is made for the preventing of wear and tear, first, by the polifh of the cartilaginous furfaces, fecondly, by the healing lubrication of the mucilage; and, in part, to that astonishing property of animal conftitutions, affimilation, by which, in every portion of the body, let it confift of what

it will, fubftance is reftored, and wafte re

paired.

Moveable joints, I think, compose the curiofity of bones; but their union, even where no motion is intended or wanted, carries marks of mechanism and of mechanical wifdom. The teeth, efpecially the front teeth, are one bone fixed in another like a peg driven into a board. The futures of the skull are like the edges of two saws clapped together, in such a manner as that the teeth of one enter the intervals of the other. We have fometimes one bone lapping over another, and planed down at the edges; fometimes also the thin lamella of one bone received into a narrow furrow of another. In all which varieties we seem to discover the same design, viz. firmness of juncture, without clumsiness in the feam.

CHAPTER IX.

OF THE MUSCLES.

MUSCLES, with their tendons, are the inftruments by which animal motion is performed. It will be our bufinefs to point out inftances in which, and properties with respect to which, the difpofition of these muscles is as ftrictly mechanical, as that of the wires and strings of a puppet.

I. We may obferve, what I believe is uni-verfal, an exact relation between the joint and the mufcles which move it. Whatever motion, the joint, by its mechanical conftruction, is capable of performing, that motion, the annexed mufcles, by their pofition, are capable of producing. For example; if there be, as at the knee and elbow, a hinge joint, capable of motion only in the fame plane, the leaders, as they are called, i. e. the mufcular tendons, are placed in directions parallel to the bone, fo as, by the contraction or relaxation of the mufcles to which they belong, to produce that motion and no other. If these

joints were capable of a freer motion, there are no muscles to produce it. Whereas at the fhoulder and the hip, where the ball and focket joint allows by its construction of a rotatory or sweeping motion, tendons are placed in fuch a pofition, and pull in fuch a direction, as to produce the motion of which the joint admits. For inftance, the fartorius or taylor's muscle, rifing from the spine, running diagonally across the thigh, and taking hold of the infide of the main bone of the leg a little below the knee, enables us, by its contraction, to throw one leg and thigh over the other giving effect, at the fame time, to the ball and focket joint at the hip, and the hinge joint at the knee. There is, as we have feen, a specific mechanism in the bones for the rotatory motions of the head and hands: there is, alfo, in the oblique direction of the muscles belonging to them, a specific provision for the putting of this mechanism of the bones into action. And mark the confent of uses. The oblique mufcles would have been inefficient without the articulation: the articulation would have been loft, without the oblique muscles. It may be proper however to observe with respect to the head, although I think

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