fyftem. This affigns 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 neceffary; and four are accordingly provided: two, called ventricles, which Send out the blood, viz. one into the lungs, in the first inftance; the other into the mafs, after it has returned from the lungs : two others alfo, 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 fecond time after its cir culation through the lungs. So that there are two receiving cavities, and two forcing cavi ties. The ftructure of the heart has reference to the lungs, for without the lungs one of each would have been fufficient. The tranf lation of the blood in the heart itself is after this manner. The receiving cavities refpectively 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.

The 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 fhort and popular account do this), but it is abundantly fufficient to teftify 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 conclufion.

"The wisdom of the Creator," saith Hamburgher, "is in nothing feen more gloriously

than in the heart." And how well doth it execute its office! An anatomift, who underftood the structure of the heart, might say 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 foon work itself out. Yet fhall this wonderful machine go, night and day, for eighty years together, at the rate of a hundred thousand ftrokes every twenty-four hours, having, at every stroke, a great refiftance to overcome; and fhall continue this action for this length of time, without disorder and without weariness.

But further; from the account, which has been given of the mechanifm of the heart, it

is evident that it must require the interposition of valves; that the fuccefs indeed of its action must depend upon these, for when any one of its cavities contracts, the neceffary 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 courfe 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 cafes, is to fix valves; which, like flood-gates, may open a way to the ftream in one direction, and fhut up the paffage against it in another. The heart, conftituted as it is, can no more work without valves, than a pump can. When the pifton descends in a pump, if it were not for the ftoppage by the valve beneath, the motion would only thruft down the water which it had before drawn up. A fimilar confequence would fruftrate the action of the heart. Valves therefore properly difpofed, i. e. properly with respect to the courfe of the blood which

A

it is neceffary to promote, are effential to the contrivance. And valves fo difpofed are, accordingly, provided. A valve is placed in the communication between each auricle and its ventricle, left, when the ventricle contracts, part of the blood fhould get back again into the auricle, inftead of the whole entering, as it ought to do, the mouth of the artery. valve is alfo fixed at the mouth of each of the great arteries which take the blood from the heart: leaving the paffage free, fo long as the blood holds its proper course forward; closing it, whenever the blood, in confequence of the relaxation of the ventricle, would attempt to flow back. There is fome variety in the con ftruction of thefe valves, though all the valves of the body act nearly upon the fame princi ple, and are destined to the fame ufe. In ge neral they confift of a thin membrane, lying clofe to the fide of the veffel, and confequently allowing an open paffage whilft the stream runs one way, but thrust out from the fide by the fluid getting behind it, and oppofing the paffage of the blood, when it would flow the other way. Where more than one membrane is employed, the different membranes only compofe one valve.

Their joint action fulfills the office of a valve: for inftance; over the entrance of the right auricle of the heart into the right ventricle, three of these skins or membranes are fixed; of a triangular figure; the bases of the triangles fastened to the flesh; the fides and fummits loose; but, though loofe, connected by threads of a determinate length with certain fmall fleshy prominences adjoining. The effect of this conftruction is, that, when the ventricle contracts, the blood endeavouring to escape in all directions, and amongst other directions preffing upwards, gets between these membranes and the fides of the paffage; and thereby forces them up into fuch a pofition, as that, together, they conftitute, when raifed, a hollow cone (the ftrings, before spoken of, hindering them from proceeding or feparating further); which cone, entirely occupying the paffage, prevents the return of the blood into the auricle. A fhorter account of the matter may be this: So long as the blood proceeds in its proper courfe, the membranes which compose the valve are preffed close to the fide of the veffel, and occafion no impediment to the circulation; when the blood would regurgitate, they are raised from the fide of the veffel,

and