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focket joint at the elbow, which admits of motion in all directions, might, in fome degree, have answered the purpose, of both moving the arm, and turning the hand. But how much better it is accomplished by the prefent mechanism, any person may convince himself, who puts the ease and quickness, with which he can fhake his hand at the wrist circularly (moving likewise, if he please, his arm at the elbow at the fame time), in competition with the comparatively flow and laborious motion, with which his arm can be made to turn round at the shoulder, by the aid of a ball and focket joint.

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III. The pine or back bone is a chain of joints of very wonderful conftruction. Various, difficult, and almoft inconfiftent offices. were to be executed by the fame inftrument. It was to be firm, yet flexible (now I know no chan made by art, which is both thefe for by firmness I mean, not only ftrength, but ftability); firm, to fupport the erect pofition of the body; flexible, to allow of the bending of the trunk in all degrees of curvature. It was further alfo, which is another, and quite a diftinct purpose from the reft, to become a pipe or conduit for the fafe conveyance from the brain of the most important fluid of the

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animal frame, that, namely, upon which all voluntary motion depends, the spinal marrow; a fubftance, not only of the first neceffity to action, if not to life, but of a nature fo delicate and tender, so susceptible and so impatient of injury, as that any unusual preffure upon it, or any confiderable obftruction of its course, is followed by paralyfis or death. Now the fpine was not only to furnish the main trunk for the paffage of the medullary fubftance from the brain, but to give out, in the course of its progrefs, fmall pipes therefrom, which, being afterwards indefinitely subdivided, might, under the name of nerves, diftribute this exquifite fupply to every part of the body. The fame fpine was also to serve another ufe not lefs wanted than the preceding, viz. to afford a fulcrum, ftay, or bafis (or more properly speaking a series of these) for the insertion of the muscles which are spread over the trunk of the body; in which trunk there are not, as in the limbs, cylindrical bones, to which they can be faftened, and, likewife, which is a fimilar ufe, to furnish a fupport for the ends of the ribs to rest

upon.

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Befpeak of a workman a piece of mechanism

which fhall comprise all these purposes, and let him fet about to contrive it; let him try his fkill upon it; let him feel the difficulty of accomplishing the tafk, before he be told how the fame thing is effected in the animal frame. Nothing will enable him to judge fo well of the wisdom which has been employed: nothing will difpofe him to think of it fo truly. First, for the firmness, yet flexibility, of the spine, it is compofed of a great number of bones (in the human fubject of twentyfour) joined to one another, and compacted together by broad bafes. The breadth of the bafes upon which the parts feverally reft, and the clofeness of the junction, give to the chain its firmness and ftability: the number of parts, and confequent frequency of joints, its flexibility. Which flexibility, we may also obferve, varies in different parts of the chain: is leaft in the back, where ftrength more than flexure is wanted: greater in the loins, which it was neceflary fhould be more fupple than the back; and greatest of all in the neck, for the free motion of the head. Then, fecondly, in order to afford a paffage for the descent of the medullary fubftance, each of these bones is bored through in the middle in such a man

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ner, as that, when put together, the hole in one bone falls into a line, and correfponds, with the holes in the two bones contiguous to it. By which means, the perforated pieces, when joined, form an entire, close, uninterrupted channel: at least whilft the spine is upright and at reft. But, as a fettled posture is inconfiftent with its ufe, a great difficulty ftill remained, which was to prevent the vertebræ fhifting upon one another, fo as to break the line of the canal as often as the body moves or twifts; or the joints gaping externally, whenever the body is bent forward, and the spine, thereupon, made to take the form of a bow. Thefe dangers, which are mechanical, are mechanically provided against. The vertebræ, by means of their proceffes and projections, and of the articulations which fome of these form with one another at their extremities, are fo locked in and confined, as to maintain, in what are called the bodies or broad surfaces of the bones, the relative pofition nearly unaltered; and to throw the change and the preffure, produced by flexion, almoft entirely upon the intervening cartilages, the springiness and yielding nature of whose subftance admits of all the motion which is neceffary

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neceffary to be performed upon them, without any chaẩm being produced by a separation of the parts. I fay of all the motion which is neceffary; for although we bend our backs to every degree almoft of inclination, the motion of each vertebra is very small; fuch is the advantage which we receive from the chain being compofed of fo many links, the fpine of fo many bones. Had it confifted of three or four bones only, in bending the body the fpinal marrow muft have been bruised at every angle. The reader need not be told that these intervening cartilages are griftles; see them in perfection in a loin Their form alfo favors the fame intention. They are thicker before than behind, fo that, when we ftoop forward, the compreffible fubftance of the cartilage, yielding in its thicker and anterior part to the force which fqueezes it, brings the furfaces of the adjoining vertebræ nearer to the being parallel with one another than they were before, instead of increasing the inclination of their planes, which must have occafioned a fiffure or opening between them. Thirdly, For the medullary canal giving out in its courfe, and in a convenient order, a supply of nerves to different parts

and he of veal.

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