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acts. Whether it be irritation excited by the contact of the blood, by the influx of the ner.j vous fluid, or whatever else be the cause of its motion, it is, something which is capable of producing, in a living muscular fibre, reciprocal contraction and relaxation. This is; the; power we have to work with: and the . enquiry is, how this power is applied in the for stance before us. There is provided in. the central part of the body a hollow muscle, invested with spiral fibres, running in both dj-> rectioas, the layers intersecting one another; in some animals, however, appearing to be, semicircular rather than spiral. By the con traction of these fibres, the sides of the muscular cavities are necessarily squeezed together, so as to force out from them any fluid which, they may at that time contain : by the relaxation of the same fibres, the cavities are is their turn dilated; and, of course, prepared to admit every fluid which may be poured into them. Into these cavities are inserted the great trunks, both of the arteries which carry out the blood, and of the veins, which bring it back. This is a general account of the apparatus: and the simplest idea of its action is, that, by each contraction, a portion of blood

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is forced as by a syringe into the arteries; ind, at each dilatation, an equal portion is received from the veins. This produces, at each pulse, a motion and change in the mass of blood, to the amount of what the cavitv contains, which in a full grown human heart, I understand, is about an ounce, or two

. tabldUpoorts full. How quickly these changes succeed one another, and by this succession how sufficient they are to support a stream or circulation throughout the system, may be understood by the following computation, abridged from KeilPs Anatomy, p. 117, ed. 3. ** Each ventricle will at least contain one ounce of blood. The heart contracts four thoufand times in one hour; from which it follows, that there passes through the heart, every hour, four thousand ounces, or three hundred and fifty pounds of blood. Now the whole mass of blood is faid to be about twenty-five pounds, so that a quantity of blood equal to the whole mass of blood passes through the heart fourteen times in one hour; which is about once every four minutes." Consider what an affair, this is^ when we come to very large animals. The

aorta of a whale is larger in the bore than the main pipe of the water-works at London M 3 Bridge; Bridge; and the water roaring in its passage through that pipe, is inferior, in impetus and velocity, to the blood gushing from the whale's heart. Hear Dr. Hunter's account of the dissection of a whale. "The aorta measured a foot diameter. Ten or fifteen gallons of blood is thrown out of the heart at a stroke with an immense velocity, through a tube of , a foot diameter. The whole idea fills the mind with wonder *." "<

The account which we have here stated, of the injection of blood into the arteries by the contraction, and of the corresponding reception of it from the veins by the dilatation, -of the cavities of the heart, and of the circulation being thereby maintained through the blood-vessels of the body, is true, but impersect. The heart performs this office, but it is in conjunction with anorher of equal curiosity and importance. It Was necessary that the blood should be successively brought into contact, or contiguity, or proximity with the air. I do not know that the chymical reason, upon which this necessity is founded, has .... - . . , V r r'.i

* Dr. Hunter's account of the dissection of a whale. Phil. Trans.

, .. been been yet sufficiently explored. It seems to be made appear, that the atmosphere which we breathe is a mixture of two kinds of air? one pure and vital, the other, for the purposes of life, effete, foul, and noxious: that when we have drawn in our breath, the blood in the lungs imbibes from the air, thus brought into contiguity with it, a portion of its pure ingredi*ent; and, at the fame time, gives out the effete or corrupt air which it contained, and which is carried away, along with the half- tus, every time we expire. At least; by comparing the air which is breathed from the lungs, with the air before it enters the lungs, it is found to have lost some of its pure part, and to have brought away with it an addition of its impure part. Whether these experiments fatisfy the question, as to the need which the blood stands in, of being visited by continual accesses of air, is not for us to 'enquire into; nor material to our argument: it is sufficient to know, that, in the conslitution of most animals such a neceffi y exists, and that the air, by some means or other, mujl be introduced into "a near communication with the blood. The lungs of animals are constructed for this -'purpose. They consist ''l M 4 of of blood-vessels and air-vessels lying close to each other; and wherever there is a branch of the trachea or windpipe, there is a branch accompanying it of the vein and artery, and the air-vessel is always in the middle between the blood-vessels*. The internal surface of these vessels, upon which the application of the air to the blood depends, would, if collected and expanded, be, in a man, equal to a superficies of fifteen feet square. Now in order to give the blood in its course the benefit of this organization (and this is the part of the subject with which we are chiefly concerned), the following operation takes place. As soon as the blood is received by the heart from the veins of the body, and before that it is sent out again into its arteries, it is carried, by the force of the contraction of the heart, and by means of a separate and supplementary artery, to the lungs, and made to enter the vessels of the lungs; from which, after it has undergone the action, whatever it be, of that vifeus, it is brought back by a large vein once more to the heart, in order, when thus concocted and prepared, to be from thence distributed anew into the

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