THE CAUSE OF THE HEART-BEAT
The primacy of the blood was no isolated fact for Harvey, but one linked with the very existence of the circulation. This primacy depended largely upon the blood being the primal abode of the innate heat. Palpitation produced by the innate heat in the blood itself, he held to be the first sign of life in the embryo and the last sign of life in the dying creature; and a swelling produced by the innate heat, he held to take place throughout life, localized in the blood just outside of the entrance to the heart. This local swelling of the blood was, to him, the exciting cause of the heart-beat and, therefore, of the circulation. We have heard him deny that the blood possesses motion "as the gift of the heart."[205] We can now grasp the probable meaning of this denial. He would not have been illogical had he said also that the heart possesses motion as the gift of the blood. This view of the cause of the heart-beat was first set forth by Harvey in 1649 in the Exercises to Riolanus, and in immediate connection with declarations in favor of the primacy of the blood, which also was first formally advocated in those Exercises. As we know, the question of this primacy had given Harvey food for thought long before. But his view of the cause of the heart-beat is not to be found in his lecture notes, nor in the treatise of 1628, and may well have been a later outgrowth from the larger doctrine of the primacy of the blood.
Let us now turn to the Exercises and to Harvey's own account of the cause of the heart-beat. The first passage to be quoted begins with a few sentences which have been introduced previously, but which form a necessary cue for the statement we are to study. Harvey says to Riolanus:—
"For the present I will only say and set forth without demonstration—by good leave of the learned and with due respect to the ancients—that the heart, as the beginning, author, source, and origin of everything in the body and the first cause of life, should be held to include the veins and all the arteries and also the contained blood; just as the brain, including all its nerves and sensory organs and spinal marrow, is the one adequate organ of sensation, as the phrase is. If by the word 'heart,' however, only the body of the heart be meant with its ventricles and auricles, I do not believe that it is the manufacturer of the blood; nor that the blood possesses vigor, faculty, reason,[168] motion, or heat, as the gift of the heart. Moreover, I judge the cause of diastole and expansion not to be the same as that of systole and contraction, either in the arteries, or in the auricles or the ventricles of the heart; but that part of the pulse which is called diastole has another cause, different from the systole, and always and everywhere must precede every systole; I judge the first cause of expansion to be the innate heat and expansion to occur first in the blood itself, gradually thinned and swelling up like matters in fermentation, and to be extinguished last in the same; and I accept Aristotle's parallel with pottage or milk with this proviso, that the rising or falling of the blood is not brought about by vapors, or exhalations, or spirits, excited into some vaporous or aërial form, and is caused, not by an external agent, but by an internal principle, and is regulated by nature.
"Nor is the heart (like a hot kettle), as some imagine, the origin of the heat and of the blood in the same sense as a hot coal or a fire-place. The blood rather imparts heat to the heart, as to all other parts, than receives heat from it, for the blood is, of all things within the body, the hottest; and so the heart is provided with coronary arteries and veins for the same purpose as that of the arteries and veins of other parts, viz.: to secure an influx of heat which shall foster and preserve. Hence it is to use convertible terms to say that all the hotter parts contain more blood and that the richer they are in blood, the hotter they are. It is in this sense that the heart, so remarkable for its cavities, should be reckoned a workshop, source, perpetual fire-place; it is like a hot kettle by virtue, not of its body, but of its contained blood, in the same sense in which the liver, the spleen, the lungs, and other parts are reckoned hot; because they contain many veins or vessels containing blood. In this way also I maintain that the native heat, or innate warmth, being the common instrument of all the functions, is likewise the prime efficient cause of the pulse. This I do not now assert positively, but only propose as a thesis. Whatever may be brought forward to the contrary by learned and upright men without scurrilous language, clamor, or contumely, I shall be glad to know, and whoever shall do that will earn my gratitude."[206]
Harvey has thus transferred to the blood the primacy of the body, making the blood in place of the Aristotelian heart the primal abode of the innate heat, "the common instrument of all the functions." Nevertheless, the blood of the Harveian circulation cannot perform the duties of the primacy without the aid of Aristotle.
If we turn from the Exercises to the treatise On Generation, published about two years later, we find the author saying:—
"The primacy of the blood is evident from this also, that the pulse has its origin in the blood. For since a pulsation consists of two parts, to wit: an expansion and a contraction, or a diastole and systole, and since the prior of these movements is the expansion, it is plain that this action is due to the blood, but that the contraction is set a-going in the egg by the pulsating vesicle, as by the heart in the chick, by means of its own fibres as though by an instrument devised for that purpose. It is certain also that the aforesaid vesicle and, at a later time, the cardiac auricle from which pulsation starts, is excited by the blood, which expands to the motion which constricts. The diastole, I say, is produced by the blood which swells up as if with interior spirits; and so Aristotle's opinion as to the heart's pulsation—namely, that it is produced after the manner of ebullition—is in some measure true. For the same thing which we see every day in milk heated over the fire and in the fermentation of our beer, comes into play also in the pulsation of the heart, in which the blood swells as from some fermentation, is expanded, and subsides; and what is brought about in the cases aforesaid by accident and by an external agent, to wit, by adventitious heat from somewhere, is effected in the blood by the internal heat or innate spirits, and is also regulated by the soul in conformity to nature, and is kept up for the health of living things. Pulsation, therefore, is accomplished by a double agency: that is to say, the expansion or dilatation is accomplished by the blood, but the contraction or systole is accomplished in the egg by the membrane of the vesicle, in the fœtus after birth by the auricles and ventricles of the heart; and these alternate and mutually associated efforts once begun, the blood is impelled through the whole body, and thus the life of animals is perpetuated."[207]
Nearly two thousand years before Harvey's time Aristotle had said:—
"The volume of leaven[208] changes from small to great, by its more solid part becoming liquefied and its liquid, vaporized.[209] This is brought about in animals by the nature of the psychical heat, but in the case of leaven by the heat of the blended juices."[210]
Moreover, Aristotle, as Harvey says, had likened to "ebullition"[211] what Aristotle himself described as "the pulsation which occurs at the heart, at which the heart is always to be seen incessantly at work." "For," says Aristotle, "ebullition takes place when liquid is vaporized[212] by heat; for it rises up owing to its bulk becoming greater."[213] He continues:—
"In the heart the swelling up from heat of the liquid which is always arriving from the food produces pulsation, for the swelling rises against the outer tunic[214] of the heart; and this process is always and incessantly going on, for the liquid is always and incessantly flowing in, out of which the nature of the blood arises; for the blood is first worked up in the heart. The thing is plain in generation from the beginning; for before the vessels have been marked out the heart is to be seen containing blood. Hence, too, it pulsates more in the young than in the old; for the vapor[215] arises more abundantly in the young.
"All the vessels also pulsate and do so simultaneously one with another, because they are dependent upon the heart.[216] This is always moving, so that they, too, are always moving, and simultaneously one with another, when[217] the heart moves. Leaping [of the heart],[218] then, is the reaction which takes place against the condensation produced by cold, and pulsation is the vaporization[219] of heated liquid."[220]
In another treatise Aristotle says: "In all animals the blood pulsates in the vessels everywhere at the same time."[221] It is interesting, in a negative way, that his sweeping and faulty references to the pulsation of the vessels put into words no physiological idea except the vague one of "dependence" on the heart.
One may be tempted to see in the seething of the heart's blood the source of some of those spirits within the body elsewhere than in and about the heart, of which one gets brief ill-defined glimpses here and there in the genuine works of Aristotle. But no words of his can be adduced to confirm such a conjecture.
Evidently, however, the seething of the nascent blood suffices, in Aristotle's eyes, to explain both the phases of the heart-beat; for both the rising and the falling of the wall of the hot central laboratory of the blood are movements as passive apparently as those of the lid of a boiling pot. One may be excused for wondering at the crudity of such a conception; nor is one's wonder lessened by recalling that elsewhere in Aristotle's works he places at the heart the central origin of the bodily movements. But when it is recalled, as well, that Aristotle was totally ignorant of the function of muscle and, therefore, even of the mode of working of the limbs, his doctrine of the heart-beat may seem less amazing.
There are indications that the function of muscle, though unknown to Aristotle, was known not long after his time,[222] and in Galen's time that function was entirely familiar, he styling the muscles "the organs of voluntary movement," and calling their contraction their "systole," a term which has survived only in connection with the heart and arteries.[223] For Harvey, born more than thirteen centuries after Galen's death, the function of muscle was a portion of ancient knowledge; and in his treatise On the Motion of the Heart and Blood, he expressly states that the heart, including the auricles, is muscular both in structure and in function. The opinions of Harvey's day rendered these statements by no means superfluous.[224] Naturally, therefore, in accepting the aid of the Aristotelian seething of the blood in connection with the heart-beat Harvey utilized only the force of expansion thus generated, and obtained from muscle the force of contraction which he required. Indeed, the conception of the auricles and ventricles as muscular force-pumps was fundamental to his doctrine of the circulation. Moreover, we have found Harvey careful to limit and mitigate the expansion of the blood, he saying to Riolanus:—
"I accept Aristotle's parallel with pottage or milk with this proviso, that the rising or falling of the blood is not brought about by vapors, or exhalations, or spirits, excited into some vaporous or aërial form, and is caused, not by an external agent, but by an internal principle, and is regulated by nature."[225]
Long before, indeed, he had jotted down a terse statement among his lecture notes which is fatal to any extreme development of the Aristotelian idea. In dealing with the action of the heart he had written:—
"To what end? Aristotle: To none, but a passive process, as in boiling pottage. But when wounded it gives out not wind, but blood."[226]
Harvey, therefore, could do no less than criticize adversely his famous contemporary, the philosopher Descartes, for accepting in its entirety Aristotle's doctrine of the heart-beat. Referring to Descartes he says:—
"Nor in the matter of the pulse am I satisfied with the efficient cause thereof which he, following Aristotle, has laid down as the same at the systole as at the diastole, to wit: an effervescence of the blood like that produced in boiling. For the movements aforesaid are sudden strokes and swift beats; while in fermentation or ebullition nothing rises up and collapses thus, as it were in the twinkling of an eye, but there is a slow swelling with a sufficient subsidence. By means of dissection, moreover, one can discern for oneself that the ventricles of the heart are expanded as well as filled by the constriction of the auricles and are increased in size proportionately, according as they are filled more or less; and that the expansion of the heart is a movement of a certain violence, produced by impulsion, not by attraction[227] of some sort."[228]
In a letter written four years after the publication of the Exercises to Riolanus and two years after that of the treatise On Generation, Harvey sets forth anew, with admirable clearness and brevity, his doctrine as to the nature and cause of the systole of the ventricles. In this he stands upon purely modern ground as an observer, and his words are free from all Aristotelian tinge. Referring to another physiologist he says:—
"I could wish, however, that he had observed this one thing, namely, that the motion which the heart enjoys is of a threefold kind, to wit: a systole, in which the heart contracts itself and drives out the blood contained in it; and then a certain relaxation, of a character contrary to the foregoing motion, a relaxation in which the fibres of the heart which make for motion are slackened. The two motions aforesaid are inherent in the very substance of the heart, just as in all other muscles. Finally, there takes place a diastole, in which the heart is expanded by blood impelled into its ventricles out of the auricles; and the heart is incited to its own contraction by this filling and expansion of the ventricles; and the motion aforesaid always precedes the systole, which follows at once."[229]
Harvey materially clarifies his doctrine of the nature and cause of the heart-beat in the following admirable summary. In the second Exercise to Riolanus he says:—
"Since I see that many are embarrassed and doubt the circulation, and that some attack it, because they have not understood me thoroughly; for their sake I will recapitulate briefly what I meant to say in my little book on the motion of the heart and blood. The blood contained in the veins, where its deeps are, as it were, where it is most abundant, that is, in the vena cava close to the base of the heart and to the right auricle, gradually grows warm and thin by reason of its own internal heat, and swells and rises up like matters in fermentation; whereby the auricle is dilated, contracts itself by reason of its own pulsific faculty, and propels the blood promptly and frequently into the right ventricle of the heart. This, when filled, frees itself of blood at its succeeding systole by the impulsion thereof and, as the tricuspid valve is a bar to the egress of the blood, drives it where an open door is offered, into the arterial vein, and thereby brings about the expansion of the latter. The blood within the arterial vessels cannot now go back in opposition to the sigmoid valve, while at the same time the lungs are widened and enlarged and then narrowed by inspiration and expiration—and with the lungs their vessels also—and offer to the blood aforesaid a path and transit into the venous artery. The left auricle accomplishes its movement, its rhythm, its order [of events], its function, at the same time and in the same way as the right auricle, and in like manner sends on into the left ventricle out of the vessels aforesaid the same blood which the right auricle had sent on into the right ventricle. As a result the left ventricle, at the same time and in the same way as the right, impels the blood into the cavity of the aorta and consequently into all the branches of the artery, the return of the blood whence it had come being prevented in the same way as before by the barrier of an opposing valve. The arteries are filled by this sudden impulsion and, as they cannot unload themselves as suddenly, are expanded, receive an impulse, and undergo their diastole."[230]
Harvey seems to have attributed more importance to the auricular systoles than do the physiologists of to-day, he making the ventricles depend very greatly for their charge of blood upon the systole of the auricles. This view appears in three passages already quoted; and is tersely put by Harvey when he says elsewhere that the heart "is dilated by the auricle, contracts of itself";[231] that "the auricles are prime movers of the blood."[232] The unduly high value set by him upon the auricular systole agrees well with the polemical vigor with which Harvey exalted impulsion and rejected suction,[233] in his general physiology as well as in the physiology of the heart. In the heart especially the force of suction had played for centuries a part which Harvey rejected more completely than the physiologists of to-day feel warranted in doing. Again he shall speak for himself, saying tersely:—
"Hence it is made plain how the blood enters the ventricles; not by reason of being drawn in, or of the heart expanding, but because sent in by the pulse of the auricles."[234]
"The expansion of the heart," he has told us already, "is a movement of a certain violence, produced by impulsion, not by attraction of some sort." He says that he maintains these views
"against the commonly received opinion; because neither the heart nor anything else can so expand itself that it can draw anything into itself in its diastole, unless as a sponge does which has first been forcibly compressed and is returning to its natural state."[235]
But, one may ask oneself, how does that modified seething in the vena cava which produces the diastole of the right auricle produce the diastole, the simultaneous diastole, of the left auricle? In his lecture notes Harvey had stated, as Columbus had before him, that the venous artery does not pulsate—at least, he means, not in the same sense as the auricle, or ventricle, or artery.[236] Obviously regarding the left auricle there could be available, for Harvey, no explanation parallel to that of to-day, viz.: the swift conduction of a stimulus from point to point of the texture of a wall which is common to both auricles. He is careful to state that corresponding auricular events occur simultaneously and in the same way in the two auricles; and incidentally but frankly he confesses ignorance of the reasons why, in the following passage:—
"From those who declare the causes and reasons of all things in such a smattering way, I would be glad to learn how it is that both eyes move together hither and thither and in every direction when they look; how it is that this eye does not turn by itself in that direction, that eye in this; likewise, both auricles of the heart; and so forth."[237]
The circulation of the blood, then, according to the final view of its discoverer, is maintained by a self-regulating mechanism worked by causes operating within the blood itself, the "principal part" of the body. The systolic muscular contractions of the walls of the ventricles are caused by direct mechanical stimulation (in modern language) due to diastolic distension by blood of the relaxed muscular walls of these chambers. The blood which distends the ventricles is driven forcibly into them by the auricular systole, the muscular walls of the auricles having been stimulated to contract by diastolic distention due likewise to blood.
So much of Harvey's doctrine of the heart-beat, although not that of to-day, is very effective as physiology, and has advanced with modern swiftness far beyond that of his predecessors. It seems strange, therefore, even to one familiar with the movement of the Renaissance, to be swept back nearly two thousand years under Harvey's guidance to reach the underlying cause of the phenomena. According to him the distention which stimulates the right auricle to contract is produced by an expansion of the blood of the great veins, due to the innate heat. The Harveian heart-beat is caused and initiated by an Aristotelian swelling up of the hot blood. Both this expansion and the fiery central hearth at which it is produced have been expelled by Harvey from within the fully developed heart; and the primal abode of the innate heat has been transferred to the blood, with which that heat has been intimately incorporated by him. Just without the heart, moreover, Harvey has established anew the Aristotelian seething; making this the result of what we to-day may style a localized automatism of the conjoined heat and blood. He has localized this automatism of the hot blood "in the vena cava, close to the base of the heart and to the right auricle," i.e., close to that region at and between the mouths of the two venæ cavæ of our present terminology, where the physiology of to-day places, not within the blood but in the texture of the walls which contain it, the seat of what is prepotent in determining the rhythm of the mammalian heart-beat.
Observation shows that from seemingly pulseless peripheral veins the blood continuously enters the venæ cavæ, which pulsate visibly in the region of Harvey's swelling of the blood. Yet in his lecture notes, in dealing with the significance of the thick resistent walls of the arterial vein and the aorta, he wrote: "Neither the vena cava nor the venous artery is of such construction, because they do not pulsate but, rather, are attracted."[238] On a neighboring page he had written:—
"At the same time [that] the pulse of the artery is perceived by touch, the vena cava is attracted, as it were."[239]
We will not now search for what he meant by saying that "the vena cava is attracted, as it were." Clearly, however, in denying that it pulsates, he meant not to deny that its wall moves rhythmically, but to deny only that this movement is of the nature of what he styles pulsation in the case of the auricles or the ventricles, or the arteries, or the arterial vein.
We know not what influence the rhythmic movements of the wall of the vena cava may have had upon Harvey's transfer to its cavity of the Aristotelian seething of the blood. To this was referable the palpitation seen by him in the blood itself as the first sign of life in the embryo and the last sign of life in the dying animal; and in this same familiar seething he found ready to his hand a life-long cause for the visible sharp expansion of the auricle in its diastole, for which expansion he could find no such obvious muscular cause as for the corresponding expansion of the ventricle or the arteries. The seething of the blood, however, was carefully kept by him below the point of vaporization and adapted to maintain the circulation by keeping the muscular cardiac pump at work.
Connected with Harvey's doctrine of the cause of the heart-beat there is a point which a student of his thought may find knotty, despite the aid of a well-developed historical sense. Harvey made the systolic contraction of auricle or ventricle dependent on the mechanical stimulus of its next preceding diastolic distension. It is not quite easy to see how he found this process compatible with the orderly recurrence of all the systolic contractions in the beating of a nearly empty heart. It is well known that the heart may beat for a while when cut out of the body, when, therefore, the heart is nearly drained of blood. In the treatise of 1628 Harvey himself speaks of studying the ventricular systole of "the heart of an eel, taken out and laid upon the table or the hand"; and says that the phenomena seen in this are seen likewise "in the hearts of little fishes, and in those colder animals in which the heart is conical or elongated."[240] In his lecture notes he says, we remember, that "the auricles pulsate after removal of the heart, because of the multitudinous blood."[241] But this jotting, written only as a brief reminder for himself, is obscure to others. By the word "heart" Harvey means sometimes the ventricular mass without the auricles and sometimes the ventricular mass and the auricles taken together. Hence it is uncertain whether the above reference be to auricles left attached to the body or removed with the ventricular mass. In neither case is it easy to imagine effective distention produced by the seething even of "the multitudinous blood." However, in the same lecture notes a few pages farther on Harvey says: "Nevertheless, the heart pulsates, cut away from the auricles;"[242] and in the treatise of 1628 he says:—
"The heart of the eel and of some fishes, and of animals even, when taken out, pulsates without auricles; indeed, if you cut it in pieces you shall see its divided parts contracting and relaxing separately; so that in these creatures the body of the heart pulsates and palpitates after the auricles have ceased to move. Is this, however, peculiar to the animals which are more tenacious of life, whose radical moisture is more glutinous, or rich, and sticky,[243] and not so readily dissolved? For in eels the thing is apparent even in their flesh, which retains the power of motion after they have been skinned, drawn, and cut in pieces."[244]
At this point we may recall the following words of our author:—
"I affirm also that in this way the native heat or innate warmth, being the common instrument of all the functions, is likewise the prime efficient cause of the pulse."[245]
Should we hazard the improbable guess that Harvey meant his cause of the heart-beat to be effective only in warm-blooded animals, we must remind ourselves that it certainly was well known to him as to all the other physicians of his day that the heart of the mammal beats after excision. If few had made experiments, all had studied Galen; and Galen cites the beating of the heart after excision as evidence that its beat does not depend upon the nervous system, the context making it obvious that he refers to the heart-beat of the mammal. Moreover, he makes it evident that the striking phenomenon in question must have been seen by the ancients at the altar, as an incident of sacrificial rites.[246] This fact makes it easy to understand how it happened that earlier still, at least two centuries before Galen's time, the layman Cicero, one of Harvey's favorite authors, should have made a stoic say:—
"It has often been observed that, when the heart of some animal has been torn out, it palpitates with a mobility which imitates the swiftness of fire."[247]
Moreover, thirty-five years before Harvey was born, even the beating of the excised human heart had been seen by Vesalius, and referred to in his celebrated treatise on anatomy, as an incident of one of the barbarous executions of the sixteenth century.[248]
By no means in accord with the cause of the heart-beat first advocated by Harvey in 1649, is an experiment which he himself had brought forward in support of the circulation in 1628. In the famous treatise of that year he tells us that if the vena cava of a living snake be compressed at a point some distance away from the heart, the vein between that point and the heart is nearly emptied by the heart-beat, and the heart itself becomes paler and shrinks from lack of blood "and at length beats more languidly."[249] These words show that in this experiment the orderly heart-beats must have continued after the blood remaining in the vena cava had become too scanty to excite them by its expansion in accordance with his doctrine. It is, therefore, an interesting question how Harvey could reconcile the beating of the empty heart with his belief as to the "prime efficient cause" of its beat.