Pagina-afbeeldingen
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parts of the body, notches are made in the upper and lower edge of every vertebra; two on each edge; equidiftant on each fide from the middle line of the back. When the vertebræ are put together, these notches, exactly fitting, form fmall holes; through which the nerves, at each articulation, iffue out in pairs, in order to fend their branches to every part of the body, and with an equal bounty to both fides of the body. The fourth purpofe affigned to the fame inftrument, is the infertion of the bafes of the muscles, and the fupport of the ends of the ribs and for this fourth purpose, especially the former part of it, a figure, specifically suited to the design, and unnecessary for the other purposes, is given to the conftituent bones. Whilft they are plain, and round, and smooth towards the front, where any roughness or projection might have wounded the adjacent vifcera, they run out, behind, and on each fide, into long proceffes, to which proceffes the mufcles neceflary to the motions of the trunk are fixed; and fixed with fuch art, that, whilst the vetebræ fupply a bafis for the mufeles, the muscles help to keep these bones in their position, or by their tendons to tie them togetherjo in una

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That most important, however, and general property, viz. the ftrength of the compages, and the fecurity against luxation, was to be still more ípecially confulted; for where so many joints were concerned, and where, in every one, derangement would have been fatal, it became a fubject of ftudious precaution. For this purpose, the vertebræ are articulated, that is, the moveable joints between them are formed, by means of those projections of their subftance, which we have mentioned under the -name of proceffes; and thefe fo lock in with, and overwrap, one another, as to secure the body of the vertebra, not only from accidentally flipping, 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 ftructure in the chine of a hare. In this, as in many inftances, a plain obferver of the animal œconomy may spare himself the difguft of being prefent at human diffections, and, yet learn enough for his information and fatisfaction, 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; confifting,

we will suppose of three vertebræ. He will find the middle bone of the three, fo ?mplicated, by means of its projections or proceffes, with the bone on each fide of it, that no preffure which he can ufe, will force it out of its place between them. It will give way neither forward, nor backward, nor on either fide. In whichever direction he pushes, he perceives, in the form, or junction, or overlapping of the bones, an impediment oppofed to his attempt; a check and guard against diflocation. In one part of the fpine, he will find a still further fortifying expedient, in the mode according to which the ribs are annexed to the spine. Each rib refts upon two vertebræ. 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 furfaces, which furfaces are joined to the bodies of two contiguous vertebræ, 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 fame purpose of ftability; viz. the cheeks of the bars, which pass between the arches, ride

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across the joints, by which the pieces compofing each arch are united. Each crofs bar refts 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 flip out of its place. Thus perfectly, by one means or the other, is the danger of flipping laterally, or of being drawn aside out of the line of the back provided against : and, to withstand the bones being pulled afunder longitudinally, or in the direction of the line, a ftrong membrane runs from one end of the chain to the other, fufficient to refift any force which is ever likely to act in the direction of the back, or parallel to it, and confequently to fecure 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 fafety, but that it is an accident hardly ever heard of, that even the gefticulations of a harlequin distort his spine.

Upon the whole, and as a guide to thofe who may be inclined to carry the confideration of this fubject further, there are three views under which the spine ought to be regarded, and in all which it cannot fail to excite our admiration.

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These views relate to its articulations, its liga ments, and its perforation; and to the corre fponding advantages which the body derives from it, for action, for ftrength, and for that, which is effential to every part, a secure communication with the brain.

The structure of the fpine is not in general different in different animals. In the ferpent tribe, however, it is confiderably varied; but with a ftrict reference to the conveniency of the animal. For, whereas in quadrupeds the number of vertebræ is from thirty to forty, in the ferpent it is nearly one hundred and fifty: whereas in men and quadrupeds the furfaces of the bones are flat, and thefe flat furfaces laid one against the other, and bound tight by finews; in the ferpent, the bones play one within another like a ball and focket that they have a free motion upon one another in every direction: that is to fay, in men and quadrupeds firmnefs is more confulted; in ferpents, pliancy. Yet even pliancy is not obtained at the expense of safety. The backbone of a ferpent, for coherence and flexi bility, is one of the moft curious pieces of

Der. Phyf. Theol. p. 396.

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