First step in the discovery—Animal Heat the product of Respiration. Second step—Heat evolved in the lungs by Respiration there produces Expansion. Third step—Expansion; implied motion, which from the organism must conduct the blood to the left ventricle of the Heart. Theory imperfect, until the formation of sufficient vapor or steam in the lungs is perceived and acknowledged.

To Dr.Marcy.—In complying with your request to write for your journal an article embodying my theory of the motive powers which produce the circulation of the blood, together with some account of its rise and progress, I obey what I regard as a call of duty; and thus requested, do it with pleasure.

But my theory, with its history, cannot thus be written without egotism. Logicians say, that the way to convince others is to retrace, in order, the steps by which you yourself became convinced, which is to be egotistic. But in this case, there is a further reason: the scientific discoverer must speak of the apparatus by which he experiments, and mine was often my own physical frame.

Twenty years ago, while yet my mind, laboring with this great subject, was condemned

“to drudge
Without a second and without a judge,”

you, sir, comprehended the hypothesis which has now become a theory, and you waited not for others to speak, but you fully acknowledged its truth; and although, in Hartford, as now in New-York, you were thronged with practice, (then allopathic), you yet found time to furnish me with added experiments, made in your office, confirmatory of its truth, which by your permission were afterwards added in your name to my published work.

The first step in the theory occurred to my mind in the winter of 1822, and while I was engaged in founding the Troy Female Seminary. Being in attendance on a course of lectures on chemistry, and at the same time teaching to a class Mrs.Marcett’s excellent work on that subject, one cold morning, as I was walking briskly up a hill, I said to myself, Why do I grow warm? Whence comes this accession of caloric? It cannot be transmitted to me from any object without, because every thing which comes in contact with me is cold. Snow is under my feet, and frosty air surrounds me; and, as to clothing, even the softest furs impart no warmth—they but keep from escaping that which comes from within. What other method besides transmission is there of gaining heat? There is the elimination of caloric, when, in substances chemically combining, weight is gained and bulk is lost. Is there any such combination going on in me? Yes; this atmospheric air, when I inspire it, has oxygen combined with nitrogen; but when I expire, the oxygen has disappeared, and heavier substances—carbonic acid gas and watery vapor—are returned in its place. Thus, it must be, animal heat is evolved. It is the product of respiration; and it is because I breathe faster and deeper, that more carbon is oxidized or burned, and more heat is set free in my lungs; and therefore I grow warm as I walk up this hill, though all around me is cold.

The mind, excited by new and great thoughts, works with unwonted energy; and mine at once collected so many proofs, that I became perfectly convinced of the truth of the hypothesis. In searching books, I found that Lavoisier had taught the same; but he dying, his doctrine was discarded by English chemists, Dr.Black leading the way, and therefore it did not then appear in English systems of chemistry. But from that time, I cherished it with a mother’s devotion, watched changes in my own physical frame relating to it, taught it to my pupils, and held warm disputes with the medical faculty, who opposed and contemned it.

In the summer of 1832, the Asiatic cholera appeared among us, appalling every heart. This plague, I said, is a disease of coldness and obstruction; and these doctors, wrong as they are on the subject of animal heat, can never understand it—though, if Lavoisier were living, he might. Let me, then, as best I may, consider anew the problem of heat as produced by respiration, and see whether I cannot find out something which has a bearing on the fatal coldness of this fearful disease. It is into the lungs, and no where else, that breathing introduces atmospheric air; and it is there that the oxidation of carbon or animal combustion takes place. Thus must caloric be imparted to the blood in the lungs; and in them is one-fifth of the blood of the system, of which seven-eighths is water.

The nature of heat is to expand all fluids. The blood in the lungs must, therefore, expand; and if it expands, it must move; and if it moves, it must, from the organism of the parts, move to the left ventricle of the heart, into which the valvular system opens to give it a free passage—whereas the valves of the right close against it. “Eureka!” I mentally exclaimed; “I have found the primum mobile of the circulation of the blood.” I had for years disbelieved that the heart’s slight mechanical impulse was that cause. In teaching Paley’s “Natural Theology,” my mind had come in contact with the passage in which he describes the heart’s more than Herculean labors; and I said, “This is altogether too much—the heart alone cannot perform all this—there must be some other power,” and an abiding desire to know what that power could be, prepared me for receiving this great idea. But my mind was agitated by it, as the sea is, when a great rock is thrown into its waters.

The cholera was then raging around me; and as I prepared to flee from it to a mountain air, I confided to a scientific friend, Professor Twiss of West Point, my hypothesis, which I regarded as probably the incipient germ of an important discovery.

But there was first the former theory to be disproved; and then there were new points to be investigated and established. In the ensuing winters of 1833, 4, and 5, I gave much attention to the subject, and employed professors in my school in the departments of chemistry and natural philosophy, who assisted me,—particularly by their ingenuity in the construction of such simple pieces of apparatus as were needed.

Thus we proved that, although the heart’s action gives pulsation, it does not necessarily give circulation. By an endless india-rubber tube, filled with water, coiled upon a table and struck repeatedly at one point, a pulsation was produced throughout, but no circulation. By affixing the tube to a vessel of water, and laying it on an inclined plane, the water ran through it in an equable current, making circulation with pulsation. Clasping the hand upon the tube in successive contractions, the fluid passed on per saltem, producing circulation and pulsation united, but no acceleration of the current. Now, add valves to the tube on each side of the opening hand, and you will have the current—which is moving by gravitation, accelerated by the hand’s impulse, as the blood’s current, first moved by respiration, undoubtedly is by the heart’s beat.

The heart we regard as the grand regulator of the blood’s flow; and it is admirably situated for measuring out a regular portion of blood at every contraction. John Bell, believing in the Harveian theory, said, “It is awful to think of the unfixed position of the heart;” and Dr.Arnott declared that “the heart, the heart alone, is the ragged anomaly in the laws of fitness in mechanics.” The heart was now seen to have a right position; for it should swing loose that its moorings be not endangered; and, as whatever impugns the Creator’s unerring wisdom must be wrong, so the presumption is, that whatever vindicates it must be right.

My hypothesis assumed the principle, that, if an endless hollow tube be filled with a liquid, the liquid can be made to circulate perpetually, if it be heated at one point and cooled before its return. A drawing of the simple apparatus by which this problem was proved, is given in my published work on “the Motive Powers, &c.” The figure which represents this apparatus gives the learner the most simple idea possible of the connection of the respiratory and circulatory systems, and of the combination of the two motive powers; the first, or chemical, coming from the lungs, and the second, or mechanical, from the heart.

Suppose the heart divided into right and left hearts by dissection at the septum: the circulatory system might then be represented by an endless tube. Let such an one, nine or ten feet in length, and of one inch bore (to be filled with water) be placed upon a horizontal table. Let an enlargement of the tube be made by a tin vessel to represent the lungs, which shall contain about one-fifth part of the water. Let the tube connected with the right side of the vessel have, at a little distance from the vessel, a smaller enlargement, composed of india-rubber, which can be grasped by the hand, to represent the heart’s right ventricle, with a valve on each side opening towards the tin vessel, the two to represent the tricuspid and semi-lunar valves. Let the whole be made nearly full of water; then, under the tin vessel (representing the lungs), let a fire be made. As the water heats, it will expand; and as the valve closes to the right, it will go off to the left side of the vessel. But, as no water will come in from the right, on account of the valves, there will be no current. Now let the hand grasp the india-rubber, and the fluid between the valves being displaced by its pressure, all the water will go towards the tin vessel, because, while the valve representing the tricuspid would close, that representing the semi-lunar (between the mimic heart and lungs) would open—and very freely; because the expansion made by the heat under the tin vessel had created a vacuum, and thus made a suction power to draw it forward, while there is a driving power behind to force it onward into the tin vessel. Then relaxing the hand, a vacuum will exist between the two valves; and the valve in the rear of the current now begun (the tricuspid) would open, and the water rush in to fill the vacuum in the india-rubber ventricle, to be again pressed forward by the next grasp of the hand; and thus—the fire (representing respiration) being kept up, and the alternate grasping and relaxing of the hand (representing the heart’s regular impulse)—a perpetual circulation might be made to go on;[1] but not without another condition of the problem.

And it was in performing this experiment that a truth was discovered, which, had it been known, many who have ignorantly lost their lives might have preserved them. When the fluid in the apparatus became equally, or nearly as much, heated at the extreme parts of the circulating tube as at the heating vessel, then the motive power of expansion ceased, and (the hand’s impulse being too weak of itself to carry it on) circulation failed; but it was restored by putting snow or ice around the extreme parts of the tube. How often have we heard of ladies who, having gone into warm baths, have been found dead by their friends, or too nearly so, to be restored.[2] Through ignorance of the cause, no right means would be taken to restore them, such as dashing cold water upon the exterior, with simultaneous efforts to produce, in fresh air and in proper position, such artificial respiration as leads to the natural. Where no internal lesions have occurred, there is every reason to believe that such measures might produce restoration.

My imperfect machines gave me to see how much might be done for this important part of physiology by a more perfect apparatus. Mine was merely horizontal—but one might be made to take as many positions as are natural to the human frame; and how many facts might such an one elicit concerning the effects of position on the circulation, by which lives might every day be saved! But skilful mechanicians, not ordinary mechanics, are needed, who are men of intellectual capacity, and are furnished with carte-blanche for time and expense.[3]

The years 1836-’7-’8 witnessed, on my part, several extraordinary and fruitless efforts to get before the public the theory, of whose truth and importance I was then fully convinced. In 1839, Dr.C. Smith, then of Troy, an able medical lecturer, became a convert to the theory; and showed me, in post mortem dissections, the organs of respiration and circulation. At the close of that year, having carefully corrected and made out copies of my manuscript theory, which I had before written, I sent two to Paris—one to the two brothers, Drs.Edwards, members of the French Institute, and one to my friend, Madame Belloc. I also sent one to Edinburgh, to Dr.Abercrombie. Dr.Milne Edwards soon after wrote a book, in which he made it a point to show that animals could live several minutes without breathing; and Dr. Frederic Edwards wrote me a short letter of objections to my theory, and adherence to that of Harvey. This letter was copied and answered in my work published in 1846.

About this time, Dr.Aikin, of Baltimore, wrote to me on the subject; and showed, by calculations, that the mere gradual expansion of the water of the blood was not sufficient, of itself, to produce a current as rapid as that of the blood was proved to be, even on the lowest estimate of its velocity. This did not shake my faith in the great fact that circulation was created by respiration. It must be so; for in life, such respiration as produces heat is the invariable antecedent of circulation, and nothing else is. There was something, then, which remained to be discovered. Again, I placed before me the conditions of the great problem, and set myself intently to its study; and I soon found what I thus sought, and then discerned for the first time that the blood moves, as does the railroad car, by steam. John Bell, my favorite author, had shown that the lungs work in vacuo. A great proportion of the blood is water, which, in a vacuum, springs into vapor at 67°, and the temperature of the blood in the lungs is 101°. Its expansion, then, was not merely the gradual increase of bulk by transmitted heat, but also that of instantaneous expansion, by the vaporization of so much of it as is needed; and what expanded water could not do, steam certainly could. At once, a throng of proofs came to my mind. The most apparent of these was the vapor expired breathing. I recollected how, in former times, the stage horses, driven rapidly into my native village of a winter morning, had clouds of vapor wreathing upward from their nostrils, while the icicles of condensation were hanging below. The nurse, who stands over the dying, holds a mirror before the mouth and nose, and considers that life is only extinct when vapor ceases to be formed. Then came to mind the solution of that great mystery of physiology, why the arteries are empty at death, which so long hindered the discovery finally made by Harvey.

In the state in which chemistry was, even as late as the time of John Bell, the chemical power of the heat produced by respiration at the lungs could not have been understood.