whole mass of blood is said 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 in 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 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 are 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 imperfect. The heart performs this office, but it is in conjunction with another 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 chemical reason, upon which this necessity is founded, has 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 a contiguity with it, a portion of its pure ingredient, and at the same time gives out the effete or corrupt air which it contained, and which is carried away, along with the halitus, every time we expire. At least, by comparing the air which is breathed from the lungs, with the air which enters the lurgs, 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 satisfy the question, as to the need which the blood stands in of being visited by continual

* Dr. Hunter's Account of the Dissection of a Whale. (Phil. Trans.)

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The next thing to be considered, is the engine which works this admirable machinery, viz.

The Heart.

For our present purpose, it is unnecessary to ascertain the principle upon which the heart acts. Whether it be irritation, excited by the contact of the blood, by the influx of the nervous fluid, or whatever else may 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 inquiry is, how this power is applied in the instance before us. There is provided, in the central part of the body, a hollow muscle, invested with spiral fibres, running in both directions, the layers intersecting each other; in some animals, however, appearing to be semicircular rather than spiral. By the contraction 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 in 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 is forced by a syringe into the arteries; and, 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 cavity contains, which, in a full-grown human heart, I understand, is about an ounce, or two table-spoons full. How quickly these changes succeed one another, and, by this succession, how capable they are to support a stream or circulation throughout the system, may be understood by the following computation, abridged from Keill's Anatomy, p. 117, ed. 3: "Each ventricle will contain at least one ounce of blood. The heart contracts four thousand times in one hour; from which it follows, that there pass through the heart, every hour, four thou sand ounces, or three hundred and fifty pounds of blood. Now the

whole mass of blood is said 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 in 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 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 mea❤ sured a foot diameter. Ten or fifteen gallons of blood are 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 imperfect. The heart performs this office, but it is in conjunction with another 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 chemical reason, upon which this necessity is founded, has 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 a contiguity with it, a portion of its pure ingredient, and at the same time gives out the effete or corrupt air which it contained, and which is carried away, along with the halitus, every time we expire. At least, by comparing the air which is breathed from the lungs, with the air which 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 satisfy the question, as to the need which the blood stands in of being visited by continual

• Dr. Hunter's Account of the Dissection of a Whale. (Phil. Trans.)

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accesses of air, is not for us to inquire into, nor material to our purpose it is sufficient to know, that in the constitution of most animals, such a necessity exists, and that the air, by some means or other, must be introduced into a near communication with the blood. The lungs of animals are constructed for this purpose. They consist 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, the following operation takes place. As soon as the blood is received by the heart from the veins of the body, and before that is sent out again into the 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 viscus, it is brought back by a large vein, once more to the heart, in order, when thus concocted and prepared, to be thence distributed anew into the 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 send 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 same 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 reference 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

* Keill's Anatomy, p. 121.

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 same blood into the mouths of the arteries.

"The wisdom of the Creator," saith 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 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 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-four 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.

The Lungs

The lungs are composed of two lobes, one seated on each side of the mediastinum. Each of these lobes is subdivided into two or three lobules, a kind of structure distinguishable in such animals as have much motion in their back, and for the same end that the liver is in the same animals. Their substance is composed of some very small cells, which are the extremities of the trachea arteria, or bronchos. The figure of these cells is irregular; yet they are fitted to each other so as to have common sides, and leave no void space. Within these cells there is secreted a quantity of mucus, and also a large quantity of the materia perspirabilis, which at once keeps the lungs from being dried by the air, and produces that necessary discharge from the blood which also takes place by perspiration through the skin. Throughout the whole texture of these cells are distributed the branches of the pulmonary artery and vein, which ramify to an infinite degree of minuteness, and supply the last-mentioned secretions.

The pulmonary artery and vein, however, are not intended for the nourishment of the lungs; but the blood, in its passage through