Harvey's Views on the Use of the Circulation of the Blood

by John Green Curtis

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 But the expression "animal spirits" was for too long a time an English technical term to be superseded now.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]