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socket joint at the elbow, which admits of motion in all directions, might, in some degree, have answered the purpose, of both moving the arm, and turning the hand. But how much better it is accomplished by the present mechanism, any person may convince himself, who puts the ease and quickness, with which he can shake 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 socket joint.
III. The spine or back bone is a chain of joints of very wonderful construction. Various, difficult, and almost inconsistent offices were to be executed by the fame instrument. It was to be firm, yet flexible (now I know no chan made by art, which is both these; for by firmness I mean, not only strength, but stability) ; Jim> to support the erect position of the body ;flexible, to allow of the bending of the trunk in all degrees of curvature. It was further also, which is another, and quite a distinct purpose from the rest, to become a pipe or conduit for the fase conveyance from the brain of the most important fluid of the animal frame, that, namely, upon which all voluntary motion depends, the spinal marrow; a substance, not only of the first necessity to action, if not to life, but of a nature so delicate and tender, so susceptible and so impatient of injury, as that any unusual pressure upon it, or any considerable obstruction of its course, is followed by paralysis or death. Now the spine was not only to furnish the main trunk for the passage of the medullary substance from the brain, but to give out, in the course of its progress, small pipes therefrom, which, being asterwards indefinitely subdivided, might, under the name of nerves, distribute this exquisite supply to every part of the body. The same spine was also to serve another use not less wanted than the preceding, viz. to afford a fulcrum, stay, or basis (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 fastened, and, likewise, which is a similar use, to furnish a support for the ends of the ribs to rest upon. „. .
Bespeak of a workman a piece of mechanism which shall comprise all these purposes, and let him set about to contrive it; let him try his skill upon it; let him feel the difficulty of accomplishing the task, before he be told how the fame thing is effected in the animal frame. Nothing will enable him to judge so well of the wisdom which has been employed: nothing will dispose him to think of it so truly. First, for the firmness, yet flexibility, of the spine, it is composed of a great number of bones (in the human subject of twentyfour) joined to one another, and compacted together by broad bases. The breadth of the bases upon which the parts severally rest, and the closeness of the junction, give to the chain its firmness and stability: the number of parts, and consequent frequency of joints, its flexibility. Which flexibility, we may also observe, varies in different parts of the chain: is least in the back, where strength more than flexure is wanted: greater in the loins, which it was necessary should be more supple than the back; and greatest of all in the neck, for the free motion of the head. Then, secondly, in order to afford a passage for the descent of the medullary substance, each of these bones is bored through in the middle in such a manner, ner, as that, when put together, the hole in one bone falls into a line, and corresponds, 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 whilst the spine is upright and at rest. But, as a settled posture is inconsistent with its use, a great difficulty still remained, which was to prevent the vertebræ shifting upon one another, so as to break the line of the canal as often as the body moves or twists; or the joints gaping externally, whenever the body is bent forward, and the spine, thereupon, made to take the form of a bow. These dangers, which are mechanical, are mechanically provided against. The vertebra, by means of their processes and projections, and of the articulations which some of these form with one another at their extremities, are so locked in and confined, as to maintain, in what are called the bodies or broad surfaces of the bones, the relative position nearly unaltered; and to throw the change and the pressure, produced by flexion, almost entirely upon the intervening cartilage; the springiness and yielding nature of whole substance admits of all the motion which is , . necessary necessary to be performed upon them, without any chasm being produced by a separation of the parts. I fay of all the motion which is necessary ; for although we bend our backs . to every degree almost of inclination, the motion of each vertebra is very small; such is the advantage which we receive from the chain being composed of so many links, the spine of so many bones. Had it confided of three or four bones only, in bending the body the spinal marrow must have been bruised at every angle. The reader need not be told that these intervening cartilages are gristles; and he may see them in persection in a loin of veal. Their form also favors the fame intention. They are thicker before than behind, so thar, when we stoop forward, the compressible substance of the cartilage, yielding in its thicker and anterior part to the force which squeezes it, brings the surfaces 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 occasioned a fissure or opening between them. . Thirdly, For the medullary canal giving out in its course, and in a convenient order, a supply of nerves to different