I should have sent letters to you sooner, but our public troubles in part, and in part the labour of putting to press my work “On the Generation of Animals,” have hindered me from writing. And indeed I, who receive your works—on the signal success of which I congratulate you from my heart—and along with them most kind letters, do but very little to one so distinguished as yourself in replying by a very short epistle. I only write at this time that I may tell you how constantly I think of you, and how truly I store up in my memory the grateful remembrance of all your kindnesses and good offices to myself and to my nephew, when we were each of us severally in Florence. I would wish, illustrious sir, to have your news as soon as convenient:—what you are about yourself, and what you think of this work of mine; for I make no case of the opinions and criticisms of our pretenders to scholarship, who have nothing but levity in their judgments, and indeed are wont to praise none but their own productions. As soon as I know that you are well, however, and that you live not unmindful of us here, I propose to myself frequently to enjoy this intercourse by letter, and I shall take care to transmit other books to you. I pray for many and prosperous years to your Duke; and for yourself a long εὐημερία. Farewell, most learned sir, and love in return.
Yours, most truly,
William Harvey.
The 15th of July, 1651.
LETTER IV
In reply to R. Morison, M.D., of Paris
Illustrious Sir,—The reason why your most kind letter has remained up to this time unanswered is simply this, that the book of M. Pecquet, upon which you ask my opinion, did not come into my hands until towards the end of the past month. It stuck by the way, I imagine, with some one, who, either through negligence, or desiring himself to see what was newest, has for so long a time hindered me of the pleasure I have had in the perusal. That you may, therefore, at once and clearly know my opinion of this work, I say that I greatly commend the author for his assiduity in dissection, for his dexterity in contriving new experiments, and for the shrewdness which he still evinces in his remarks upon them. With what labour do we attain to the hidden things of truth when we take the averments of our senses as the guide which God has given us for attaining to a knowledge of his works; avoiding that specious path on which the eyesight is dazzled with the brilliancy of mere reasoning, and so many are led to wrong conclusions, to probabilities only, and too frequently to sophistical conjectures on things!
I further congratulate myself on his confirmation of my views of the circulation of the blood by such lucid experiments and clear reasons. I only wish he had observed that the heart has three kinds of motion, namely, the systole, in which the organ contracts and expels the blood contained in its cavities, and next, a movement, the opposite of the former one, in which the fibres of the heart appropriated to motion are relaxed. Now these two motions inhere in the substance of the heart itself, just as they do in all other muscles. The remaining motion is the diastole, in which the heart is distended by the blood impelled from the auricles into the ventricles; and the ventricles, thus replete and distended, are stimulated to contraction, and this motion always precedes the systole, which follows immediately afterwards.
With regard to the lacteal veins discovered by Aselli, and by the further diligence of Pecquet, who discovered the receptacle or reservoir of the chyle, and traced the canals thence to the subclavian veins, I shall tell you freely, since you ask me what I think of them. I had already, in the course of my dissections, I venture to say even before Aselli had published his book,[57] observed these white canals, and plenty of milk in various parts of the body, especially in the glands of younger animals, as in the mesentery, where glands abound; and thence I thought came the pleasant taste of the thymus in the calf and lamb, which, as you know, is called the sweetbread in our vernacular tongue. But for various reasons, and led by several experiments, I could never be brought to believe that that milky fluid was chyle conducted hither from the intestines, and distributed to all parts of the body for their nourishment; but that it was rather met with occasionally and by accident, and proceeded from too ample a supply of nourishment and a peculiar vigour of concoction; in virtue of the same law of nature, in short, as that by which fat, marrow, semen, hair, &c., are produced; even as in the due digestion of ulcers pus is formed, which the nearer it approaches to the consistency of milk, viz. as it is whiter, smoother, and more homogeneous, is held more laudable, so that some of the ancients thought pus and milk were of the same nature, or nearly allied. Wherefore, although there can be no question of the existence of the vessels themselves, still I can by no means agree with Aselli in considering them as chyliferous vessels, and this especially for the reasons about to be given, which lead me to a different conclusion. For the fluid contained in the lacteal veins appears to me to be pure milk, such as is found in the lacteal veins [the milk ducts] of the mammæ. Now it does not seem to me very probable (any more than it does to Auzotius in his letter to Pecquet) that the milk is chyle, and thus that the whole body is nourished by means of milk. The reasons which lead to a contrary conclusion, viz. that it is chyle, are not of such force as to compel my assent. I should first desire to have it demonstrated to me by the clearest reasonings, and the guarantee of experiments, that the fluid contained in these vessels was chyle, which, brought hither from the intestines, supplies nourishment to the whole body. For unless we are agreed upon the first point, any ulterior, any more operose, discussion of their nature, is in vain. But how can these vessels serve as conduits for the whole of the chyle, or the nourishment of the body, when we see that they are different in different animals? In some they proceed to the liver, in others to the porta only, and in others still to neither of these. In some creatures they are seen to be extremely numerous in the pancreas; in others the thymus is crowded with them; in a third class, again, nothing can be seen of them in either of these organs. In some animals, indeed, such chyliferous canals are nowhere to be discovered (vide Liceti Epist. xiii, tit. ii, p. 83, et Sennerti Praxeos, lib. v, tit. 2, par. 3, cap. 1); neither do they exist in any at all times. But the vessels which serve for nutrition must necessarily both exist in all animals, and present themselves at all times; inasmuch as the waste incurred by the ceaseless efflux of the spirits, and the wear and tear of the parts of the body, can only be supplied by as ceaseless a restoration or nutrition. And then, their very slender calibre seems to render them not less inadequate to this duty than their structure seems to unfit them for its performance: the smaller channels ought plainly to end in larger ones, these in their turn in channels larger still, and the whole to concentrate in one great trunk, which should correspond in its dimensions to the aggregate capacity of all the branches; just such an arrangement as may be seen to exist in the vena portæ and its tributaries, and farther in the trunk of the tree, which is equal to its roots. Wherefore, if the efferent canals of a fluid must be equal in dimensions to the afferent canals of the same fluid, the chyliferous ducts which Pecquet discovers in the thorax ought at least to equal the two ureters in dimensions; otherwise they who drink a gallon or more of one of the acidulous waters could not pass off all this fluid in so short a space of time by these vessels into the bladder. And truly, when we see the matter of the urine passing thus copiously through the appropriate channels, I do not see how these veins could preserve their milky colour, and the urine all the while remain without a tinge of whiteness.
I add, too, that the chyle is neither in all animals, nor at all times, of the consistency and colour of milk; and therefore did these vessels carry chyle, they could not always (which nevertheless they do) contain a white fluid in their interior, but would sometimes be coloured yellow, green, or of some other hue (in the same way as the urine is affected, and acquires different colours from eating rhubarb, asparagus, figs, &c.); or otherwise, when large quantities of mineral water were drunk, they would be deprived of almost all colour. Besides, did that white matter pass from the intestines into those canals, or were it attracted from the intestines, the same fluid ought certainly to be discovered somewhere within the intestines themselves, or in their spongy tunics; for it does not seem probable that any fluid by bare and rapid percolation of the intestines could assume a new nature, and be changed into milk. Moreover, were the chyle only filtered through the tunics of the intestines, it ought surely to retain some traces of its original nature, and resemble in colour and smell the fluid contained in the intestines; it ought to smell offensively at least; for whatever is contained in the intestines is tinged with bile, and smells unpleasantly. Some have consequently thought that the body was nourished by means of chyle raised into attenuated vapour, because vapours exhaling in the alembic, even from fœtid matters, often do not smell amiss.
The learned Pecquet ascribes the motion of this milky fluid to respiration. For my own part, though strongly tempted to do otherwise, I shall say nothing upon this topic until we are agreed as to what the fluid is. But were we to concede the point (which Pecquet takes for granted without any sufficient reason in the shape of argument), that chyle was continually transported by the canals in question from the intestines to the subclavian veins, in which the vessels he has lately discovered terminate, we should have to say that the chyle before reaching the heart was mixed with the blood which is about to enter the right side of the organ, and that it there obtains a further concoction. But what, some one might with as good reason ask, should hinder it from passing into the porta, then into the liver, and thence into the cava, in conformity with the arrangement which Aselli and others are said to have found? Why, indeed, should we not as well believe that the chyle enters the mouths of the mesenteric veins, and in this way becomes immediately mingled with the blood, where it might receive digestion and perfection from the heat, and serve for the nutrition of all the parts? For the heart itself can be accounted of higher importance than other parts, can be termed the source of heat and of life, upon no other grounds than as it contains a larger quantity of blood in its cavities, where, as Aristotle says, the blood is not contained in veins as it is in other parts, but in an ample sinus and cistern, as it were. And that the thing is so in fact, I find an argument in the distribution of innumerable arteries and veins to the intestines, more than to any other part of the body, in the same way as the uterus abounds with blood-vessels during the period of pregnancy. For nature never acts inconsiderately. In all the red-blooded animals, consequently, which require [abundant] nourishment, we find a copious distribution of mesenteric vessels; but lacteal veins we discover in but a few, and even in these not constantly. Wherefore, if we are to judge of the uses of parts as we meet with them in general and in the greater number of animals, beyond all doubt those filaments of a white colour, and very like the fibres of a spider’s web, are not instituted for the purpose of transporting nourishment, neither is the fluid they contain to be designated by the name of chyle; the mesenteric vessels are rather destined to the duty in question. Because, of that whence an animal is constituted, by that must it necessarily grow, and by that consequently be nourished; for the nutritive and augmentative faculties, or nutrition and growth, are essentially the same. An animal, therefore, naturally grows in the same manner as it receives immediate nutriment from the first. Now it is a most certain fact (as I have shown elsewhere) that the embryos of all red-blooded animals are nourished by means of the umbilical vessels from the mother, and this in virtue of the circulation of the blood. They are not nourished, however, immediately by the blood, as many have imagined, but after the manner of the chick in ovo, which is first nourished by the albumen, and then by the vitellus, which is finally drawn into and included within the abdomen of the chick. All the umbilical vessels, however, are inserted into the liver, or at all events pass through it, even in those animals whose umbilical vessels enter the vena portæ, as in the chick, in which the vessels proceeding from the yelk always so terminate. In the selfsame way, therefore, as the chick is nourished from a nutriment, (viz. the albumen and vitellus,) previously prepared, even so does it continue to be nourished through the whole course of its independent existence. And the same thing, as I have elsewhere shown, is common to all embryos whatsoever: the nourishment, mingled with the blood, is transmitted through their veins to the heart, whence moving on by the arteries, it is carried to every part of the body. The fœtus when born, when thrown upon its own resources, and no longer immediately nourished by the mother, makes use of its stomach and intestines just as the chick makes use of the contents of the egg, and vegetables make use of the ground whence they derive concocted nutriment. For even as the chick at the commencement obtained its nourishment from the egg, by means of the umbilical vessels (arteries and veins) and the circulation of the blood, so does it subsequently, and when it has escaped from the shell, receive nourishment by the mesenteric veins; so that in either way the chyle passes through the same channels, and takes its route by the same path through the liver. Nor do I see any reason why the route by which the chyle is carried in one animal should not be that by which it is carried in all animals whatsoever; nor indeed, if a circulation of the blood be necessary in this matter, as it really is, that there is any need for inventing another way.