RESPIRATION AND THE CIRCULATION
So the feeding of the tissues could not sufficiently account, to Harvey's mind, for the swiftness of the circulation. What could? It is easy for us to recite the multitudinous modern duties of the blood as a bearer of cells and of chemicals from point to point and as a protector against poisoning; above all it is easy to exclaim "respiration";—to read the most striking part of the riddle by knowing the answer which was wrung laboriously from Nature after Harvey had died. It is easy for us to see that speedy death from loss of the circulating blood is practically the same as death from ligature of the arteries of the brain, or from drowning, or strangulation, or a broken neck. But this was veiled from him, and what best accounts for the volume and swiftness of the Harveian circulation was, in Harvey's day, a stumbling block to its acceptance; for no adequate reason was apparent why the whole mass of the blood should traverse the lungs, or why, if the veins receive their blood from the arteries, the venous blood should differ in color from the arterial.
Let us remember that throughout Harvey's life air was still an elementary body in the eyes of many and, for all, blood was a quite mysterious, ruddy, hot, vital liquid. Only weak magnifying glasses were available for him, and the powerful lenses of Malpighi and van Leeuwenhoek had not yet revealed to the world either capillary or blood-corpuscle. Moreover, the gossiping John Aubrey, the man who had been advised about his youthful studies by Harvey, wrote of him some years after his death, that "he did not care for Chymistrey, and was wont to speake against them [the chemists] with an undervalue."[19] Where would physiology be to-day, had not histology and chemistry long stood in the forefront beside her?
In a passage of the treatise of 1628 Harvey speaks of respiration, as follows:—
"And now it has come to this, that it would seem better worth while and more straightforward for those who seek the path by which in man the blood passes through the vena cava into the left ventricle and the venous artery,[20] to be willing to search for the truth by dissecting animals, in order to look for the reason why in the larger and more perfect animals, when full grown, nature chooses to make the blood percolate through the parenchyma of the lungs rather than take wide open paths as in all other animals (it being understood that no other path and transit can be thought out):—whether it is because the larger and more perfect animals are hotter and when they are full grown their heat is more ignited, so to speak, and prone to be smothered, that there is this permeation and transfer through the lungs in order that the heat may be tempered by the inspired air and guarded from boiling up and smothering—or for some other similar reason. But to determine these matters and explain them completely were to enter on a speculation as to the purpose for which the lungs are made. About these and their use and motion, and the whole subject of ventilation and the need and use of air, and other matters of this sort, and about the various different organs created in animals by reason thereof, although I have made a vast number of observations, I shall not speak till I can more conveniently set them forth in a treatise apart, lest by wandering at this point too far from my subject, which is the motion and use of the heart, I should seem to deal with something else and leave my position, to confuse and evade the question."[21]
Farther on in the same treatise Harvey says:—
"Moreover, the reason why the lungs have vessels so ample, both vein and artery, that the trunk of the venous artery exceeds in size the crural and jugular branches taken both together; and the reason why the lungs are so full of blood as we know them to be by experience and inspection (heeding Aristotle's warning,[22] and not deceived by the inspection of such lungs as we have removed from dissected animals from which all the blood had flowed out)—the reason is, that in the lungs and heart is the storehouse, the source, the treasury of the blood, the workshop of its perfection."[23]
So the great Englishman gropes for a moment or two by the light of ancient Greek doctrines and puts the question of respiration by. But this very attitude shows Harvey's thought to be in such contrast with the thought of to-day that in order to understand him we need to learn more fully his views of respiration; and we find with satisfaction that in his lecture notes of more than eleven years before he had not put this question by, for he had been called upon to lecture upon the uses of the lungs. We must seek in his lecture notes, therefore, for what he had thought those uses to be. These notes, however, we shall be unable to follow unless now, first of all, we shall give the floor for a while to the ancients; for from their doctrines Harvey necessarily took his cue, like the other thinkers of his time.
The momentous physiological facts that the living body of man, beast, or bird, is warm of itself and that its cooling means its death, must always have struck and impressed the human mind, whether trained or untrained. More than nineteen centuries before Harvey certain thoughts of Aristotle were recorded as follows:—
"In animals all the parts and the entire body possess a certain innate natural heat; wherefore they are sensibly warm when living, the reverse when making an end and parting with life. In the animals which have blood the origin of this heat is necessarily in the heart, in the bloodless kinds in the analogue thereof; for all work up and concoct the nourishment by means of the natural heat, the master part most of all. Life persists, therefore, when the other parts are chilled; but if what resides in this one be so affected total destruction ensues, because upon this part they all depend as the source of their heat, the soul being as it were afire within this part; that is, within the heart in the animals which have blood, in the bloodless kinds in the analogue thereof. Necessarily, therefore, the existence of life is coupled with the preservation of the heat aforesaid, and what is called death is the destruction thereof."[24]
This heat which is innate in all living animals was styled by Aristotle not only "innate" but "natural," "vital,"[25] and "physical,"[26] it being indispensable to life and to the working of the soul. He held the continued existence of the innate heat to depend upon conditions similar to those under which a fire is kept alive, viz.: protection both from burning out and from extinction due to external forces. Yet the true nature of combustion was not settled till more than a century after Harvey's death. The fact that air is necessary to fire must always have been a matter of common knowledge. Therefore, the views of the relations of air to fire maintained by Aristotle nearly twenty-one centuries before the discovery of oxygen did not seem naïve to Harvey, whatever they may seem to us. Aristotle held that air exerts upon fire a cooling influence which saves it from burning out too fast; and that the same influence is exerted upon the vital innate heat of animals by the air which they breathe in, or the water which bathes their gills.[27] Moreover, Aristotle says:—
"Why those animals breathe most which have lungs containing blood, is plain from this: that the warmer an animal is, the greater need it has of cooling, while at the same time the breath passes easily toward the source of warmth within the heart. But the way in which the heart is pierced through toward the lung must be studied from dissections and from the history of animals which I have written. In general terms, then, it is the nature of animals to need cooling on account of the firing of the soul within the heart."[28]
In the treatise styled the "History of Animals," to which he refers us, Aristotle says:—
"There are also channels from the heart which lead into the lung and divide in the same way as the windpipe, and they accompany the channels from the windpipe throughout the entire lung. The channels from the heart lie uppermost; but no common channel exists, for it is by contact[29] that they receive the breath and transmit it to the heart."[30]
The collection of ancient Greek commonly called the "Works of Hippocrates" is judged to be of the fifth and fourth centuries B.C. There is included in this collection a brief treatise on the heart; and in this occurs the earliest known account of the structure and use of the semilunar valves, which together with the rest of the cardiac valves were unknown to Aristotle. In the same Hippocratic treatise the doctrine is adhered to of the entrance of air into the heart for cooling purposes, both the right and the left ventricle being specified as receiving it. The author says:—
"The vessel which leads out of the right ventricle ... closes toward the heart, but closes imperfectly, in order that air may enter, though not very much."[31]
This piece of incorrect physiology may well have received support from the fact that the pulmonary semilunar valve is commonly found to be not quite competent when the dead and dissected pulmonary artery of the bullock is distended with water—an observation which the ancient author intimates that he has made,[32] though he does not specify the creature dissected.
Nearly five hundred years after the death of Aristotle, the analogy between life and flame was discussed, formally and at some length, by Galen. He knew his Aristotle well, and agreed with him as to the importance of respiratory cooling for protracting the indispensable heat of animals.[33] But we find Galen dealing with the uses of respiration in a less simple way than Aristotle. In a polemical treatise Galen debates the question whether "the breath drawn in in respiration" actually enters the heart, or whether it cools it without entering it. He says:—
"It is possible that the whole is breathed out again, as was believed by most physicians and philosophers, and those the keenest, who say that the heart, while it craves to be cooled, is in need not of the substance, but of the quality[34] of the breath, and that the use of respiration is indicated by the part.... I have shown in my treatise on the use of respiration that either an absolutely minute quantity, or none at all, of the substance of the air, is taken into the heart."[35]
It is clear, however, that Galen, when delivering himself of the foregoing, was a trifle carried away by the ardor of contention; for in the very treatise to which he refers us, as well as elsewhere, he not only dilates upon the cooling effects of breathing, but admits the entrance of air into the heart for a definite physiological purpose. This purpose, however, which we shall study later, is not cooling and is counted of secondary importance by Galen. Nevertheless, he goes so far as to say this:—
"That some portion of the air is drawn into the heart in its diastole and fills the vacuum which is produced, is sufficiently shown by the very magnitude of the dilation."[36]
In his treatise "On the Use of the Parts of the Human Body" Galen takes a more judicial tone in the following brief, calm summary:—
"The use of the respiration of animals arises from the heart, as has been shown. The heart itself needs in some sort the substance of the air; but, first and foremost, it craves to be cooled, because it boils with heat. The heart is cooled by the cool quality of inspiration; but expiration also cools, by pouring out that which seethes within the heart and is, in a way, burned up and sooty."[37]
Thus do we see the modern products of respiration foreshadowed.
Galen believed that the heat of animals is safeguarded also by the entrance of cooling air through the pores of the skin into the arterial system, and by the exit through these pores of injurious fumes out of the arteries.[38] In the introduction to Harvey's great treatise of 1628[39] the English physician riddles with adverse arguments this doctrine of Galen; to this we shall return later, as we shall to Galen's belief that the brain draws cooling air directly into its ventricles out of the nares through the cribriform plate of the ethmoid bone.[40]
In passing from Aristotle to Galen we have crossed nearly five centuries. Now let us pass at a leap across fourteen centuries more, from Galen at imperial Rome under Septimius Severus to Harvey at London under King James the First. Having briefly scanned the doctrines of the Greeks, let us take up our study of respiration in Harvey's private lecture notes of 1616. His crabbed handwriting has been deciphered by experts, and his notes have been both photographed and printed. If we seek therein for his thoughts about respiration, and track them through the jungle of abbreviated careless Latin and racy English in which they were jotted down, we shall find them Galenic in part, but also denying a truth which Galen had accepted. Harvey's notes are often too disconnected for quotation, calling rather for paraphrase or summary; and to make either is a task which one cannot approach without diffidence, especially as this task involves translation also. Of what I have ventured to prepare to represent parts of Harvey's note-book in the present paper some passages are simple translations, such English words as Harvey interspersed being transcribed. Naturally such passages are included between quotation marks. These are not used, however, in the case of a paraphrase or summary, even if it contains scattered English words which are Harvey's own.
Harvey fully shared the ancient view of the supreme importance of the heat of animals. In his note-book he, like Galen, deals with respiration under the heads: first, of a possible absorption of some of the substance of the air; and, second, of cooling and ventilation. Let us first take up the second head. Harvey says:—
"Without nourishment life cannot be, nor nourishment without concoction, nor concoction without heat, nor heat without ventilation;" for heat perishes either of wasting or of smothering; "so there is cooling and ventilation of the native heat, ventilation especially."[41]
His words contain reminders of Aristotle;[24] and he continues about respiration in a vein as ancient as Hippocrates,[42] as follows:—
"Nothing is so necessary, neither sense nor food. Life and respiration are convertible terms, for there is no life without breathing and no breathing without life. If the eye be cut out there is an end of seeing; if the legs be cut off there is an end of walking; if the tongue, of speech, et cetera; if respiration, there is an end of everything immediately."[43]
When Harvey jotted this down he had in mind a Galenic passage which doubtless had become the common property of all physicians in his day; for the removal of eye and legs figures in the first chapter of Galen "On the Use of Respiration."[44] Harvey continues:—
"Hence large animals are much warmer and breathe frequently, because they have need of greater cooling and ventilation inasmuch as they very greatly abound in blood and heat."[45]
In the margin opposite this passage there is written:—
"Why and how air is needed by animals which breathe and also air is necessary to a candle and to fire see W. H."
We may conjecture that this note refers to Harvey's promised treatise on respiration, which was never published.
So far Harvey has simply reiterated the ancient doctrine of cooling and ventilation, as in the passages quoted previously from the treatise of 1628. We shall find it very interesting to see how he deals with the other ancient doctrine that some of the substance of the air joins the blood in respiration. That this is true, gas analysis and the mercurial air-pump have taught us; but in this matter modern demonstration does but confirm, extend, and make precise one of the oldest of physiological beliefs. Regarding this we must now give the floor once again to the ancients, in order to make Harvey comprehensible.
Even in the days of Empedocles and Hippocrates, in the fifth and fourth centuries before Christ, men wrote of something derived from the outer air being present, for the use of the organism, in the vessels which also contain the blood.[46] To express this derivative of the outer air the ancient Greeks employed the word "pneuma" (πνεῦμα), the fundamental meaning of which seems to have been "air in motion." Various meanings were acquired by "pneuma," such as the breath of living things, the wind, or simply the air, or what we mean by the words "gas," "vapor," "steam," "exhalation," "emanation." The Latin word equivalent to "pneuma" is "spiritus," and so the English derivative of this, the word "spirits," came into use to express various meanings of the Greek "pneuma." A Hippocratic writer tells us that "the spirits cannot stand still, but go up and down" in the blood vessels. The word "spirits" here designates a derivative of the outer air crudely mingled with the blood.[47] To this writer the distinction between veins and arteries was unknown.
In the genuine works of Aristotle this Hippocratic doctrine does not reappear, though it is fairly certain that Hippocratic treatises which contain it were written before Aristotle's time. We have seen that the entrance of air into the heart, to cool the same, is an important feature of the Aristotelian physiology. Beyond the Aristotelian heart, however, we cannot trace the air which enters it. Yet we find "pneuma," "spirits," referred to by Aristotle, not seldom obscurely or in very general terms, as doing service, sometimes momentous service, in the physiology of generation and in certain workings within the bodies of full-grown creatures. In disease also spirits may play a very important part. These Aristotelian spirits, however, when their origin can be traced at all, are either innate or appear to be vapor produced within the body itself by heat or by disease. They do not appear to be recruited from the outer air which has penetrated the lungs and heart, that air seeming to complete its function within the lungs or within the heart itself by sustaining the native heat which is the great instrument of the soul, and in which the very soul itself is fired.[48]
Physicians of Aristotle's time, however, revived and handed on the doctrine that not only blood but a derivative of the air is distributed to the body at large through the vessels. After the distinction between veins and arteries had been clearly made and the latter had received their present name, a striking modification of this doctrine of the spirits was adopted and pressed by the Greek physician Erasistratus, about 300 B.C., not many years after the death of Aristotle. This modified doctrine separated the paths taken within the vessels by the blood and the spirits derived from the air, and declared the transmission of the necessary blood to the body at large to be by the veins only, that of the necessary spirits, styled "vital," to be by the arteries only. More than four hundred and fifty years later Galen shattered this doctrine and incorporated the vital spirits in the arteries with the blood, which he proved by epoch-making experiments to be normally present in the arteries, he, however, clearly recognizing differences between the cruder blood in the veins and the spirituous blood in the arteries. The tissues, therefore, still received vital spirits by way of the arteries, according to Galen, but not spirits in their pure gaseous Erasistratean state.[49] Now let Galen tell us more in his own words:—
"The breath from the windpipes, which had been drawn in from without, is worked up in the flesh of the lungs in the first place; in the second place in the heart and arteries, and especially in those of the net-like plexus; and to perfection in the ventricles of the brain, where the spirits become completely animal. But what the use may be of these animal spirits and why we have the temerity to call them so, when we confess that we are still utterly ignorant as to the substance of the anima [i.e., of the soul], this is not the moment to say."[50]
The complex physiology of this passage is so obsolete that its very phraseology is meaningless without a commentary. In the first place, what are the animal spirits? This expression, once a technical term of physiology, survives only in colloquial English, and even there merely as a label of which the origin is known to few. In this phrase the adjective "animal" does not refer to lower creatures as opposed to man, but is used in its obsolete original sense of "pertaining to the soul," for which latter the Latin word is "anima," the Greek word "psyche" (πσυχή). "Psychical spirits" would best translate into the English of to-day either the original Greek expression "pneuma psychikon" (πνεῦμα ψυχικόν) or its Latin equivalent "spiritus animalis." But the expression "animal spirits" was for too long a time an English technical term to be superseded now. These animal spirits, that is, spirits of the soul, were not peculiar to man, but were possessed by lower creatures also; for neither the Latin word "anima" nor the Greek word "psyche" implied immortality, as the English word "soul" is now so commonly understood to do. Plato formally recognized a mortal and an immortal part of the human psyche;[51] and Aristotle admitted the existence in animals lower than man of the lower grades of psyche, and conceded the lowest grade even to plants.[52] The perfected animal spirits were of the very highest physiological importance, as their name implies, they being for Galen no less than "the first instrument of the soul,"[53] and thus assuming the lofty rank given by Aristotle to the native heat. For Galen the animal spirits were the medium of sensation and volition and were imparted by the ventricles of the brain to the spinal cord and nerves, the fibers of which were believed, accordingly, to consist of tubes in which the subtile animal spirits were contained, the bore of these tubes being too small to be visible.
We can now follow the quoted Galenic passage and trace the full significance of that entrance of the substance of the air into the heart which Galen repeatedly acknowledged, though sometimes grudgingly. According to Galen whatever air was taken into the heart had first been "concocted" in "the flesh of the lungs." Next, this aërial substance had been worked up in the heart with the vapor of the blood into vital spirits, and these became incorporated with the finer blood destined for the arteries. Moreover, as each arterial diastole was due to an active expansion of the arterial wall, at each diastole there became blended with the contents of the arteries still more of the substance of the air, which was sucked into the arterial skin through the countless pores of the bodily skin, these being too fine to permit bleeding. The vital spirits, thus formed and modified, were blended with the blood of the arteries and supplied to the body at large. A part of these vital spirits mounted with the blood into the carotid arteries. In the swine and the ruminants, notably in the calf, the branch given to the brain by each carotid artery breaks up at the base of the skull within the cranial cavity into numerous fine twigs, which form collectively a net-work, styled in the passage from Galen already quoted the "net-like plexus." This plexus is called by modern anatomists the rete mirabile. It was falsely assumed by Galen to exist in man. The plexuses of the two sides anastomose freely across the median line, and through them passes the entire blood supply of the brain; in the animals which possess them these plexuses seem the terminal branches of the vertebral arteries also. The small vessels of each net-like plexus reunite, and thus reconstitute the artery of the brain before this artery has pierced the dura mater. Galen regarded the net-like plexus as an organ of much importance intercalated in the course of the artery for the still further elaboration of the vital spirits, which, thus altered, were exhaled from the cerebral arteries into the cerebral ventricles.[54] In these ventricles the spirits attained their final perfection, becoming "completely animal," by the aid of still more of the substance of the air, which the diastole of the pulsating brain had drawn into its cavities directly from the nares through the numerous holes in the ethmoid bones. It is a striking fact in this connection that in some of the domestic animals on each side of the head the cavity of the nares is separated from the ventricular cavity of the brain by an exceedingly thin, though complex, partition: as may be seen on dissection, if the nares and the brain in situ be opened at the same time.
Now let Galen speak again as follows:—
"I have clearly shown that the brain is, in a way, the source of the animal spirits, watered and fed by inspiration and by the abundance supplied from the net-like plexus. The proof was not so clear as to the vital spirits, but we may deem it not at all unlikely that they exist, contained in the heart and arteries, they, too, fed by respiration mainly, but to some degree by the blood also. If there be such a thing as the natural spirits, these would be found contained in the liver and veins."[55]
The animal spirits were sustained, as we have seen, by three kinds of respiration which might be called pulmonary, cutaneous and cerebral. We may perhaps conjecture that it was largely Galen's acceptance of the two latter, the last especially, which enabled him sometimes to treat as doubtful the entrance into the heart of that air from which the vital spirits were held to be derived. Of the natural spirits he evidently made small account.[56]
A modern physiologist, musing upon all this, might see in the vital spirits a dim foreshadowing of oxyhæmoglobin; might see in the operation of the animal spirits a plainer foreshadowing of the nerve impulse of to-day.
Some account, such as the foregoing, of the very complex ancient doctrine of the spirits is indispensable for the study of Harvey; for that doctrine, more or less modified, was still the accepted medical doctrine of his time. After this renewed study of the ancients let us now return again to Harvey's note-book at the place where he takes up the question of the action of the lungs upon the blood otherwise than by the cooling and ventilation of the innate heat. It is necessary in his opinion that a further concoction of the blood into spirituous arterial blood should be accomplished by the fleshy parenchyma of the lungs in animals which require a warmer, thinner, "sprightly kind of aliment," as his own English styles it.[57] The probability of such a concoction is shown by the separation of excreta which indicate it, such as sputa, at the lung.[58] On the other hand, in such creatures as frogs and turtles the lungs are fleshless, spongy, and vesicular, and give no sign of blood or excreta. Hence we may infer that the pulmonary concoction of the blood, though it probably occurs, is limited to such animals as possess fleshy and sanguinolent lungs. Hence, again, it follows that the concoction aforesaid is a function of secondary importance, because it is not universal; and that the foremost function of the lungs is their motion, the windpipes constituting their most important part, rather than the parenchyma.[59] Two functions of the lungs, says Harvey, are affirmed by the medical authorities: first, the cooling and tempering of the blood; second, the preparation of natural spirits and air to be made into vital spirits in the heart. From all this there result the excreta of pulmonary concoction, which are something between water and air, and the fumes which are breathed out in expiration continually and incessantly. Harvey observes correctly that Realdus Columbus had declared himself to have discovered the continual motion of the lung to be the means whereby the spirits are prepared; the blood being thinned by the agitation, thoroughly mixed with air, beaten, and prepared.[60] Harvey also cites Galen as saying that the parenchyma of the lung concocts spirits out of air as the flesh of the liver concocts the blood.[61] On turning to the Galenic passage cited by Harvey one finds that it is out of the food that the blood is thus concocted by the liver.
Realdus Columbus, to whom Harvey refers, was the Italian anatomist who in 1559, fifty-seven years before the Harveian circulation was verbally announced, gave to the world the important truth that such blood as the right ventricle imparts to the left reaches the latter by traversing the pores of the texture of the lungs,[62] instead of the pores of the septum of the ventricles, as Galen had taught. The existence of these pores of the septum Vesalius had pointedly wondered at in 1543 and had emphatically doubted in 1555.[63] Four years later his former assistant and temporary successor, Columbus, flatly denied the existence of the pores. It was natural, therefore, that in the same book in which Columbus brought forward the path through the lungs to replace that through the septum he should declare that the vital spirits are made out of air worked up with the blood in the lungs and then merely perfected in the left ventricle. This doctrine was an important advance beyond what Galen had taught, viz.: that the spirits are but slightly prepared in the lungs out of air and then sent to the left ventricle to undergo their main preparation and to be worked up therein with the blood which had filtered into it directly out of the right ventricle.
So much for the views of the medical authorities. We have found Harvey agreeing with them that the ancient doctrine of the cooling and ventilation of the native heat by respiration is sound. We have found him acknowledging that in some animals some sort of concoction also of the blood destined for the arteries may be brought about by the pulmonary parenchyma as a function of secondary importance. But now we shall find him rejecting the second accepted doctrine of the physicians, viz.: that some of the substance of the air is taken into the pulmonary vessels and enters the blood. This conjecture had had believers for two thousand years, and was destined to be proved true triumphantly after Harvey's death. In rejecting it he threw away a precious clue to the meaning of his own great discovery.
"It is more philosophical," he says, "not to share the common belief that the spirits are distinct and separate from the humors and parts because the spirits are produced in diverse places or contained in diverse things," but to hold that the spirits and the blood are one thing, like the cream and watery part (serum) in milk or, to borrow a simile from Aristotle's reasonings about the blood,[64] like heat and water in hot water, or like flame and a vapor which feeds it (nidor). As light is to a candle, so are the spirits to the blood.[65]
In this passage the discoverer's thought rises high, but in the next it stoops again. The next passage is headed "Spirits not from air"; and Harvey says in effect, as I understand his difficult words:—
If spirits are made by concoction out of air, the air is made either thinner or thicker in the process. If made thick, how does it get from the windpipes into the venous artery? If the spirits be thinner than air, how are they held[66] by the tunic of the lung, since this lets pass the pus and serum of empyema?[65]
In the treatise of 1628 Harvey says that Laurentius
"asserts and proves that, in empyema, serosities and pus absorbed from the cavity of the chest into the venous artery may be expelled and got rid of with the urine and fæces through the left ventricle of the heart and the arteries."[67]
Harvey's argument in his note-book continues thus:—
"How, since mixture consists in the union of altered matters, can air be thoroughly mixed and made one with blood? What is that which mixes and alters? If it be heat, the air is made thinner thereby. If it be urged that the air is thickened by cold during preparation (which is impossible in the lungs), then Aristotle's[68] argument holds good: if spirits be from the air, how about fishes, which are agile and abound in spirits?"[69]
At this point we may call to mind passages in the introduction to Harvey's treatise of 1628, published more than eleven years after he had written the notes which we are now studying. In one of these passages he speaks of what is now called the pulmonary vein, saying:—
"If it be contended that fumes and air pass to and fro by this road, as through the bronchia of the lungs, why can we find neither air nor fumes on dissection, when the venous artery has been cut out or cut into? And how comes it that we always see the aforesaid venous artery to be full of thick blood and never of air, while we perceive that there is air remaining in the lungs?"[70]
Immediately after the foregoing passage Harvey says that should an experimenter
"make a cut in the trachea of a living dog, forcibly fill the lungs with air by means of a bellows and, when they have been distended, apply a firm ligature, on opening the chest shortly after, he would find great abundance of air in the lungs, up to their outermost tunic, but none at all in the venous artery or in the left ventricle of the heart. If in the living dog the heart drew air out of the lungs or the lungs transmitted it, much more ought they to do so in this experiment. Who, indeed, could doubt that even in a dissection, if the lungs of a dead body had been inflated, air would enter at once, as aforesaid, did any passages exist?"[71]
Yet we have found Aristotle, more than nineteen centuries before Harvey, recognizing that no passages are needed for the transfer of air out of the windpipe, and saying, of the channels from the heart, that "it is by contact that they receive the breath[72] and transmit it to the heart."[73] Moreover, sixty-nine years before Harvey's publication Columbus had repeatedly recommended the experiment of opening the venous artery[20] in a living dog and noting that the "said venous artery" is full of blood, not of air or fumes. But Columbus held this observation rather to confirm than to disprove his doctrine that the blood in the venous artery is imbued with vital spirits derived in the lungs from the substance of the air. Indeed, he goes so far as to call the contents of this vessel "modified blood and air."[74] In this matter the earlier observer, Columbus, shows keener insight than the later, Harvey.
Decidedly, however, the stage waits for the chemists, despite Harvey's poor opinion of them. Despite that poor opinion, too, Harvey himself turns to making chemical conjectures in the next passage of his note-book, to the study of which latter we will now return. The passage is as follows:—
"Conclusion. Opinion of W. H.
"In animals in which lungs are fleshy and full of blood these concoct the blood, seeing that spirits and blood are one thing, in the same way that the liver does and by reason of the same arguments; indeed, the lungs may rather detain fatty and oleaginous vapor by a cooling process, as oil or balsam or nutritious fat is cooled in alembic and serpentina"[75]—
"alembic" and "serpentina" answering to the "still" and "worm" of the modern distiller. Harvey, therefore, utilizes the Galenic analogy between concoction in the lungs and that of the blood and the vapors thereof, rejecting not only Galen's preliminary concoction of air into spirits in the lungs, but also Columbus's union in the lungs of blood with spirits produced in the lungs themselves out of air. Of the entrance of "the substance of air" into the blood Harvey makes emphatic denial and, by so doing, reduces the spirits either to emanations from ingredients of the body itself (thus reminding us of Aristotle), or to a mere name with which to label qualities of the blood, in treating of which he often uses the word "spirits" as a current term. Naturally, therefore, where in his lecture notes he treats of the spirits in relation to the brain and nerves his conclusions are not clearly defined, but seem consistent with his views as to the spirits in the blood, though his jotted words are not very easy to understand. On this subject he refers by name to Galen, three alternatives discussed by whom appear to be reviewed by Harvey, viz.: that sensation and motion result either from a progression from elsewhere of spirits in substance along and within the nerves; or from a vibration of spirits in substance which have their native seat within the nerves; or, lastly, from no movement of a substance, but from a transfer of "faculty" along the nerves by means of progressive qualitative alteration thereof, "such as is produced in air by the brightness of the sun."[76] Of these three alternatives, the last seems to commend itself most to Harvey, as we should expect; the second, next; and the first, not at all;—that is, if one may so interpret the following brief passage of his lecture notes:—
"I believe that in the nerves there is no progression of spirits, but irradiation; and that the actions from which sensation and motion result are brought about as light is in air, perhaps as the flux and reflux of the sea."[77]
Also we find Harvey long years afterward saying to Riolanus:—
"Moreover, the spirits, animal, natural, vital, which dwell, contained within blind windings, in solid parts, to wit, in ligaments and nerves (especially if there be so many kinds),—these spirits are not to be regarded as so many diverse aëreal forms, nor as so many kinds of vapors."[78]
In Harvey's lecture notes the subject of respiration is brought to an end with an abrupt interrogation, which seems to reveal a sudden return of doubt as to whether too much may not have been conceded in admitting a pulmonary concoction of any sort. We read:—
"N.B. If the blood receive concoction in the lungs, why does it not traverse the lungs in the embryo?"[79]
It would seem to be Harvey's tendency to adhere to the view which limited the use of respiration entirely to the cooling and ventilation of the innate heat, by which according to ancient doctrine the heart was the central hearth, embedded in the cooling and ventilating lungs; although this ancient doctrine tallied well in most eyes with the belief that only a portion of the blood ever entered the heart at all.[80] In the first of the two Exercises which Harvey, when seventy years old, in 1649, addressed to Riolanus in defense of the circulation, the ancient respiratory cooling and ventilation take their place again as follows:—
"Thus by the aid of two extremes, viz.: cold and heat, is the temperature of the animal body retained at its mean. For as the air inspired tempers the too great heat of the blood in the lungs and centre of the body and effects the expulsion of suffocating fumes, so in its turn does the hot blood, thrown through the arteries into the entire body, cherish and nourish and keep alive all the extremities, preventing extinction due to the power of external cold."[81]
In none of the writings of his old age does Harvey deal expressly with concoction in the lungs, or more than cursorily with the entrance of the substance of air into the blood. But he repeatedly and emphatically reaffirms that blood and spirits are one thing;[82] he even declares the blood in comparison with the other parts of the body to be "possessed of powers of action beyond all the rest, and therefore, in virtue of its preëminence, meriting the title of spirit."[83] He castigates those who give the rein to overmuch speculation about the spirits. We learn that some suppose that the spirits "are engendered and are fed and increased from the thinner part of the blood"; that others suppose "the primigenial moisture" to engender and support them.[84] Then there are "those who tell us that the spirits are formed in the heart, being compounded of the vapours or exhalations of the blood (excited either by the heat of the heart or the agitation) and the inspired air"[85]—the Galenic doctrine.
"Such spirits," says Harvey of these last mentioned, "are rather to be regarded as fumes and excrementitious effluvia of the blood and body, like odours, than as natural artificers; ... whence it seems probable also that pulmonary expiration is for the ventilation and purifying of the blood by the breathing out of these; while inspiration is in order that the blood, in passing through between the two ventricles of the heart, may be tempered by the ambient cold; lest the blood, being hot and swollen, blown up in a sort of ferment, like milk and honey boiling up, should so distend the lungs that the animal would be suffocated."[86]
As we read these words, published in Harvey's old age, we recollect the following words, written in his note-book more than thirty-three years before, viz.: "So there is cooling and ventilation of the native heat, ventilation especially."[87]
We may recall also that the preservation of the native heat had sufficed to explain respiration to Harvey's ancient teacher, Aristotle, while the tenor of Aristotle's genuine works well accords with the following dictum which we have found in Harvey's note-book: "Spirits not from air." Yet the more firmly this dictum was upheld, and the more simply Aristotelian in principle did Harvey's doctrine of respiration remain, so much the less called for must have seemed that swift and endlessly repeated passage through the lungs of the whole mass of the blood, which was involved in the Harveian circulation.
In the actual phenomena of respiration, however, positive obstacles confronted the doctrine of the circulation which were harder to surmount than cobwebs of speculation, or than the mere question "cui bono" which latter the steadfast observer could simply wave aside. Spirits or no spirits, there were opponents of the circulation, even in Harvey's old age, who insisted that the blood in the arteries was so different from the blood in the veins that the same blood could not be changing perpetually from arterial to venous, and vice versa. There was always that stubborn difference of color, plainly to be seen in man and beast, but so hard to account for in Harvey's day. Therefore, we find Harvey leaving the realm of subtleties and taking up his old weapon of demonstration, in order to minimize the differences between arterial and venous blood. Twenty years after the publication of his discovery he says to Riolanus:—
"You may also perform another experiment at the same time. If you fill two cups of the same measurement with blood, one with that which issues by leaps from an artery, the other with venous blood from a vein of the same animal, you can observe the sensible differences between the two, both immediately and later, when the blood in either cup has become coagulated and cold. This experiment will contradict those who pretend that the blood in the arteries is of one kind, that in the veins of another, on the ground that that in the arteries is more florid and seethes and is blown up with copious spirits, I know not how, like milk or honey boiling upon the fire, swelling and filling a larger space. For, were the blood which is thrown from the left ventricle of the heart into the arteries fermented thus into a frothy and flatulent condition, so that a drop or two distended the whole cavity of the aorta, unquestionably, upon the subsidence of this fermentation, the volume of the blood would return to that of a few drops (and this is, indeed, the reason that some assign for the empty state of the arteries in the dead body); and this would be apparent in the cup which is full of arterial blood, for so we find it to happen in milk and honey when they come to cool. But if in both cups you find blood nearly of the same colour, not of very different consistency in the coagulated state, forcing out serum in the same manner and filling each cup to the same height when cold that it did when hot, this will be enough for any one to rest his faith upon, and afford argument enough, I think, for rejecting the dreams of certain people. On investigation sense and reason alike assure us that the blood of the left ventricle is not of a different kind from that of the right.... The blood, then, when imbued with spirits to the utmost, is not swollen with them, or fermented or blown up so as to crave and require more ample room (as can be determined with the greatest certainty on trial by the measurement of the cups); we should rather understand this blood to be possessed, after the manner of wine, of greater strength, and of an impetus to action and effectiveness, in accordance with the view of Hippocrates.
"So the blood in the arteries is the same as that in the veins; even though the former be acknowledged more spirituous and possessed of greater vital force; but the blood in the arteries is not converted into something more aëreal or rendered more vaporous; as though there were no spirits not aëreal, nor anything which gives an impetus except wind and flatulence."[88]
It is well, one may be inclined to mutter, as one reads this, but how about the color? It may be nearly the same, but certainly there is a difference. In his book "On Generation" Harvey himself describes in more detail the changes which occur in shed blood on standing, and says: "Of the red parts the upper are more florid, those below are blackish." In the same description he refers shortly after to "the florid and ruddy part which is commonly thought to be arterial blood."[89] The words last quoted evidently refer to the upper part of coagulating blood as commonly seen. This in medical practice would be blood drawn from a vein, and Harvey says nothing of arteriotomy in this passage. Indeed, he refers in the context to venesection; and earlier in the same chapter he wrote: "Physicians observe only human blood, and this shed by venesection into a basin, and coagulated."[90]
The foregoing passages show at once that opinions had been clarified very little by the suggestive change of color caused in shed blood by contact with air. Years before, in jotting down his lecture notes, Harvey had noted that the arterial blood is redder;[91] Galen had known it;[92] it must always have been known. In 1649 Harvey wrote:—
"Three things are especially apt to give rise to this opinion of the diversity of the blood: the first is that the blood which is drawn in arteriotomy is more florid....[93] Whenever and wherever blood issues through a narrow orifice it is strained, as it were, and the thinner and lighter part, which usually swims on top and is the more penetrating, is emitted."[94]
A number of observations follow, of appearances noted in nosebleed, in the use of leeches, in cupping, and in blood-letting from veins and arteries. All these appearances are adduced in support of the view that it is the straining of the blood which renders it more florid, and they all show that the brightening of the color of shed blood on exposure to air served only to lead Harvey off on a false scent. Continuing he refers, as follows, to direct inspection of the dissected lungs:—
"The blood is found to be much more florid within the lungs and after it is squeezed out of them, than in the arteries."[95]
A few pages farther on he states, categorically, the false conclusion to which he has been driven, saying:—
"It is no less plain why the blood of the lungs is so ruddy; for it is thinner, because there it is filtered through."[96]
Nothing indicates better Harvey's readiness to minimize the essential differences between venous and arterial blood than a passage in the treatise of 1628, in which he says that, compared with the left ventricle, the right ventricle "is of greater capacity, that it may supply not only matter to the left ventricle, but also nourishment to the lungs."[97] It should be remembered that, in Harvey's day, the so-called bronchial arteries were still unknown, through which the tissues of the lungs are supplied with arterial blood from the aorta.[98] Not only Columbus,[99] but even Galen,[100] had each devised an erroneous way in which to provide the lungs with "spirituous" or "vital" blood, in addition to the venous blood from the right ventricle; but Harvey is obviously content to let the latter suffice for their nutrition.
What has gone before indicates how erroneous it is to speak of the pulmonary transit, as Columbus had set it forth in 1559, nineteen years before Harvey's birth, as though Columbus were in some sort a sharer in the discovery of the circulation. Those who so speak fail to note the difference between blood and the blood. Although Columbus girded at Galen and corrected him, Columbus's pulmonary transit of a fraction of the blood by curing more than one defect of the Galenic doctrine strengthened the erroneous Galenic physiology of the blood-movement. Of these larger features Columbus not only was no enemy, but remained a devoted adherent. His doctrine certainly paved the way for Harvey's, but in no more immediate sense than did Galen's doctrine that blood is naturally contained in the arteries.[80]
Indeed, Harvey categorically stated that the movement of blood through the lungs had nothing to do with his discovery. In a Latin letter from London written in 1651 to P. M. Siegel in Hamburg, Harvey says in his old age:—
"Meantime, as Riolanus uses his utmost efforts to oppose the passage of blood into the left ventricle through the lungs, and brings it all hither through the septum, and so vaunts himself as having upset the very foundation of the Harveian circulation, (although I have nowhere laid that down as a foundation for my circulation; for the blood fetches a circuit in very many red-blooded animals in which no lungs are to be found), it may be well here to relate an experiment which I lately tried in the presence of several of my colleagues, and from the cogency of which there is no escape."[101]
The parenthesis certainly is a striking one.
No less striking is the last word published by Harvey about respiration. We have heard him deny the entrance of air into the blood and doubt the occurrence of any concoction in the lungs. Now we shall hear him throw over even the cooling of the innate heat, a respiratory doctrine to which he has seemed hitherto to hold with conviction. In the essay "On Parturition" published in 1651 with the treatise "On Generation," he says:—
"In the meantime I would propose this question to the learned: How comes it that the fœtus continues in its mother's womb after the seventh month? If brought forth at that time it breathes at will, indeed could not survive one little hour without breathing; yet, as I have said, if it remain in the womb it keeps alive and well beyond the ninth month without the aid of respiration.... Whoso shall attend carefully to these things and consider more closely the nature of air, will, I think, readily grant that air is given to animals neither for cooling nor as nutriment; for it is a fact that after the fœtus has once drawn breath it may be suffocated more quickly than when entirely excluded from the air; as though heat were unkindled by air within the fœtus rather than allayed. Thus much, merely by the way, on the subject of respiration; perhaps I shall treat of it more fully in its proper place. Surely a more knotty subject could hardly be found, as the arguments on both sides are very evenly balanced."[102]
So we find Harvey in his old age induced by lifelong study to question, if not deny, even the cooling effects of respiration, and to end with a practical confession of ignorance. Instead, therefore, of the circulation and its swiftness being explained by the urgent need of "the substance of the air" experienced by certain tissues, that movement of the whole mass of the blood through the lungs, which was so novel a physiological fact, does not seem to have affected his view of the problems of respiration. Nor could he properly explain the respiratory change in the color of the blood, which seemed to support the ancient doctrine that the blood is of two different kinds. Since he could not invoke respiration to elucidate the circulation and its rapidity, and since he himself declared that such rapidity could not be needed for the simple feeding of the tissues, what was left to be invoked? It is no wonder that eight years[103] after the publication of his discovery Harvey denied that he had ever seriously undertaken to explain the use of the circulation; that at the end of thirteen years more he repeated this denial in his old age;[104] although he had not refrained from expressing such conjectures as must always be evoked in the mind of a great observer by a discovery of the first importance made by himself. Yet the phenomena of the very circulation used were so striking as to cry aloud for elucidation; for Harvey's own clinching statement that the heart drives into the aorta at least one thousand drachms of blood in half an hour,[105] this reductio ad absurdum, which cut the ground from under the feet of his opponents, left him helpless in his turn to account for the need of so huge a flooding of the arteries.
Since it was not to be swiftly altered in the lungs that the whole mass of the blood hurried back from all parts of the chest, what then?