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system. This assigns to the heart a double office. The pulmonary circulation is a system within a system; and one action of the heart is the origin of both.
For this complicated function, four cavities become necessary; and four are accordingly provided: two, called ventricles, which fend out the blood, viz. one into the lungs, in the first instance; the other into the mass, after it has returned from the lungs: two others also, called auricles, which receive the blood from the veins; viz. one, as it comes immediately from the body; the other, as the fame blood comes a second time after its circulation through the lungs. So that there are two receiving cavities, and two forcing cavities. The structure of the heart has reserence to the lungs, for without the lungs one of each would have been sufficient. The translation of the blood in the heart itself is after this manner. The receiving cavities respectively communicate with the forcing cavities, and, by their contraction, unload the received blood into them. The forcing cavities, when it is their turn to contract, compel the fame blood into the mouths of the arteries.
This account here given will not convey to
a reader a reader ignorant of anatomy, any thing like an accurate notion of the form, action, or use of the parts (nor can any short and popular account do this), but it is abundantly sufficient to testify contrivance; and, although imperfect, being true as far as it goes, may be relied upon for the only purpose for which we offer it, the purpose of this conclusion.
"The wisdom of the Creator," faith Hamburgher, "is in nothing seen more gloriously than in the heart." And how well doth it execute its office! An anatomist, who understood the structure of the heart, might fay beforehand that it would play-: but he would expect, I think, from the complexity of its mechanism, and the delicacy of many of its parts, that it should always be liable to derangement, or that it would soon work itself out. Yet shall this wonderful machine go, night and day, for eighty years together, at the rate of a hundred thousand strokes every twenty-sour hours, having, at every stroke, a great resistance to overcome; and shall continue this action for this length of time, without disorder and without weariness.
But further; from the account, which has been given of the mechanism of the heart, it is evident that it must require the interposition of valves; that the success indeed of its action must depend upon these, for when anyone of its cavities contracts, the necessary tendency of the force will be to drive the inclosed blood, not only into the mouth of the artery where it ought to go, but also back again into the mouth of the vein from which it flowed. In like manner, when by the relaxation of the fibres the fame cavity is dilated, the blood would not only run into it from the vein, which was the course intended, but back from the artery, through which it ought to be moving forward. The way of preventing a reflux of the,fluid, in both these cases, is to fix valves; which, like flood-gates, may open a way to the stream in one direction, and shut up the passage against it in another. The heart, ct nstitured as it is, can no more work without valves, than a pump can. When the piston descends in a pump, if it were not for the stoppage by the valve beneath, the motion would only thrust down the water which it had before drawn up. A similar consequence would frustrate the action of the heart. Valves therefore properly disposed, i. e. properly with respect to the course of the blood which
it it is necessary to promote, are essential to the contrivance. And valves so disposed are, accordingly, provided. A valve is placed in the conaniunicarion between each auricle and its ventricle, lest, when the ventricle contracts, part of the blood should get back again into the auricle, instead of the whole entering, as it ought to do, the mouth of the artery. A valve is also fixed at the mouth of each of the great arteries which take the blood from the heart: leaving the passage free, so long as the blood holds its proper course forward; closing it, whenever the blood, in consequence of the relaxation of the ventricle, would attempt to flow back. There is some variety in the con struction of these valves, though all the valves of the body act nearly upon the fame principle, and are destined to the fame use. In general they consist of a thin membrane, lying close to the. side of the vessel, and don* sequently allowing an open passage whilst the stream runs one way, but thrust out from the side by the fluid getting behind it, and opposing the passage of the blood, when it would flow the other way. Where more than one membrane is employed, the different membranes only compose one valve.
Their joint action fulfills the office of a valve: for instance; over the entrance of the right auricle of the heart into the right ventricle, three of these stuns or membranes are fixed, of a triangular figure; the bases of the triangles fastened to the stem ; the sides, and summits loose; but, though loose, connected by threads of a determinate length with certain small flestiy prominences adjoining. The effect of this construction is, that, when the ventricle contracts, the blood endeavouring to escape in all directions, and amongst other directions pressing upwards, gets between these membranes and the sides of the passage; and thereby forces them up into such a position, as that, together, they constitute, when raised, a hollow cone (the strings, before spoken of, hindering them from proceeding or separating further); which cone, entirely occupying the passage, prevents the return of the blood into* the auricle. K shorter account of the matter may be this: So long as the blood proceeds in its proper course, the membranes which compose the valve are pressed close to the fide of the vessel, and occasion no impediment to the circulation; when the blood would regurgitate, they are raised from the side of the vessel,