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ing likewise, if he pleases, his arm at the elbow at the same time,) in competition with the comparatively slow 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. [Pl. IX. fig. 1, 2.] Various, difficult, and almost inconsistent offices were to be executed by the same instrument. It was to be firm, yet flexible, (now I know no chain made by art, which is both these; for by firmness I mean, not only strength, but stability:) firm, to support the erect position of the body; flewible, to allow of the bending of the trunk in all degrees of curvature. It was farther also (which is another, and quite a distinct purpose from the rest) to become a pipe or conduit for the safe 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 afterwards 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 same 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 twenty-four) joined to one another, and compacted 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 firinness and stability; the number of W. and consequent frequency of joints, its flexibility.
* It seems proper to remark here, that the form of expressio, made use of in this case implies what is not strictly true. The spinal marrow, or more properly the spinal nerve, is not a fluid but a solid cord extending from the brain down through the canal of the spine, from which branches are distributed to all parts of the body. Dr. Haley in this instance probably had in view the animal spirits, a subtile fluid, which was formerly believed to be seated in th: brain, and carried through the nerves to the different parts.-Ed.
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 the middle in such a manner, 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 vertebrae 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 vertebrae, 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 cartilages, the springiness and yielding nature of whose substance admits of all the motion which is necessary to be performed upon them, without any chasm being produced by a separation of the parts. I say, of all the motion which is necessary; for although we bend our backs to every degree almost of inclination, the motion of each vertebrae is very small: such is the advantage we receive from the chain being "omposed of so many links, the spine of so many bones
Had it, consisted 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 perfection in a loin of veal. Their form also favors the same intention. They are thicker before than behind; so that, 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 vertebrae 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 parts of the body, notches are made in the upper and lower edge of every vertebra, two on each edge, equi-distant on each side from the middle line of the back. When the vertebrae are put together, these notches, exactly fitting, form small holes, through which the nerves, at each articulation, issue out in pairs, in order to send their branches to every part of the body, and with an equal bounty to both sides of the body. The fourth purpose assigned to the same instrument is the insertion of the bases of the muscles, and the support of the ends of the ribs; and for this fourth purpose, especially the for mer part of it, a figure, specifically suited to the design, and unnecessary for the other purposes, is given to the constituent bones. Whilst they are plain, and round, and smooth, towards the front, where any roughness or projec tion might have wounded the adjacent viscera, they run out behind, and on each side, into long processes, to which processes the muscles necessary to the motions of the trunk are fixed; and fixed with such art, that, whilst the vertebrae supply a basis for the muscles, the muscles help to keep these bones in their position, or by their tendons to tie them together.
That most important, however, and general property, viz
the strength of the compages, and the security againt luxation, was to be still more specially consulted: for where so many joints were concerned, and where, in every one, derangement would have been fatal, it became a subject of studious precaution. For this purpose, the vertebrae are articulated, that is, the moveable joints between them are formed by means of those projections of their substance, which we have mentioned under the name of processes; and these so lock in with, and overwrap one another, as to secure the body of the vertebrae, not only from accidentally slipping, but even from being pushed out of its place by any violence short of that which would break the bone. I have often remarked and admired this structure in the chine of a hare. In this, as in many instances, a plain observer of the animal economy may spare himself the disgust of being present at human dissections, and yet learn enough for his information and satisfaction, by even examining the bones of the animals which come upon his table. Let him take, for example, into his hands, a piece of the clean-picked bone of a hare's back; consisting, we will suppose, of three vertebrae. He will find the middle bone of the three so implicated by means of its projections or processes, with the bone on each side of it, that no pressure which he can use, will force it out of its place between them. It will give way neither forward, nor backward, nor on either side. In whichever direction he pushes, he perceives, in the form, or junction, or overlapping of the bones, an impediment opposed to his attempt; a check and guard against dislocation. In one part of the spine, he will find a still farther fortifying expedient, in the mode according to which the ribs are annexed to the spine. Each rib rests upon two vertebrae. That is the thing to be remarked, and any one may remark it in carving a neck of mutton. The manner of it is this: the end of the rib is divided by a middle ridge into two surfaces; which surfaces are joined to the bodies of two contiguous vertebrae, the ridge applying itself to the intervening cartilage. Now this is the very contrivance which is employed in the famous iron bridge at my door at Bishop-Wearmouth; and for the same purpose of stability; viz. the cheeks of the bars, which pass between the arches, ride across the joints, by which the pieces composing each arch are united. Each cross-bar rests upon two of these pieces at their place of junction; and by that position resists, at least in one direction, any tendency in either piece to slip out of its place. Thus perfectly, by one means or the other, is the danger of slipping laterally, or of being drawn aside out of the line of the back, provided against; and to withstand the bones being pulled asunder longitudinally, or in the direction of that line, a strong membrane runs from one end of the chain te the other, sufficient to resist any force which is ever likely to act in the direction of the back, or parallel to it, ano consequently to secure the whole combination in their places. The general result is, that not only the motions of the human body necessary for the ordinary offices of life are performed with safety, but that it is an accident hardly ever heard of, that even the gesticulations of a harlequin distort his spine.
Upon the whole, and as a guide to those who may be inclined to carry the consideration of this subject farther, there are three views under which the spine ought to be regarded, and in all which it cannot fail to excite our admiration. These views relate to its articulations, its ligaments, and its perforation, and to the corresponding advantages which the body derives from it, for action, for strength, and for that which is essential to every part, a secure communication with the brain.*
*It will be useful to append to the remarks of Dr. Paley upon the mechanism of the spine and of other parts of the body, some observations by a very eminent anatomist and surgeon now living, who has lately considered the subject of Animal Mechanism in its connexion with Natural Theology, and has presented some striking and original views. These observations have been published as one of the treatises of the Society for the Diffusion of Useful Knowledge, which forms the ninth number of the series. These extracts will be the more instructive as giving views of a professional observer in confirmation of those of our author ; and they will also serve as additional illustrations of the same great truths which he has endeavoured to enforce.—Ed. “The spinal column, as it is called, serves three purposes : it is the great bond of union between all the parts of the skeleton; it forms a tube for the lodgement of the spinal marrow, a part of the nervous system as important to life as the brain itself; and lastly, it is a column to sustain the head. We now see the importance of the spine, and we shall next explain how the various offices are provided for. If the protection of the spinal marrow had been the only object of this structure, it is natural to infer that it would have been a strong and unyielding tube of bone; but, as it must yield to the inflexion of the body, it cannot be constituted in so strict an analogy with the skull. It must, therefore, bend; but it must have no abrupt or considerable bending at one part; for the spinal marrow within would in this way suffer. By this consideration we perceive why there are twenty-four bones in the spine, each bending a little; each articulated or making a joint with its fellow; all yielding in a slight degree, and, consequently, permitting in the whole spine that flexibility necessary to the motions of the body. It is next to be observed that, whilst the spine by this provision moves in every direction, it gains a property which it belongs more to our present purpose to understand. The bones of the spine are called vertebrae; at each interstice between these bones, there is a peculiar gristly substance, which is squeezed out from between the bones, and, therefore, permits them to approach and play a little in the motions of the body. This gristly substance is enclosed in an elastic binding, or membrane of great strength, which passes from the edge or border of one vertebra, to the border of the one next it. When a weight is upon the sody, the soft gristle is pressed out, and the membrane yields: the moment the weight is remov