The fifth book treats of the Holy Spirit. ‘As the essence of God is the Word,’ says our author, ‘in so far as manifestation is made in the world, so, and in so far as communication is made, it is Spirit; manifestation and communication, however, being ever co-ordinate and conjoined. It is spirit that is the architype, eternally present in God, from whom it proceeds’ (p. 163). And it is in this place that our author explains or illustrates some of his metaphysical positions by a reference to Anatomy, with which in various interesting particulars he shows himself more satisfactorily intelligible than in his transcendental speculations.
‘There is commonly said to be a threefold spirit in the body of man, derived from the substance of the three superior elements—a natural, a vital, and an animal spirit; there are, however, not really three, but only two distinct spirits. One of these, the first, characterised as natural, is communicated from the arteries to the veins by their anastomoses, and is primarily associated with the blood, the proper seat or home of which is the liver and veins. The second is the vital spirit, whose seat or dwelling-place is the heart and arteries. The third, the animal spirit, comparable to a ray of light, has its home in the brain and nerves. In each and all of these is the force—energeia—of the one spirit and light of God comprised. Now, that the natural spirit is imparted from the heart to the liver, and not from the liver to the heart, is proclaimed by the formation of man in the womb; for we see an artery associate with a vein sent from the mother through the navel of the fœtus; and in the adult body we always find an artery and a vein conjoined. But it was truly into the heart of Adam that God breathed the breath of life or the soul. From the heart, therefore, it is that life is communicated to the liver; for by the breathing into the mouth and nostrils it was that the soul was first truly imparted, the breath tending directly to the heart.
‘The heart is the first organ that lives, and, situate in the middle of the body, is the source of its heat. From the liver the heart receives the liquor, the material as it were of life, and in turn gives life to the source of the supply. The material of life is therefore derived from the liver; but, elaborated as you shall hear, by a most admirable process, it comes to pass that the life itself is in the blood—yea that the blood is the life, as God himself declares (Genes. ix.; Levit. xvii.; Deut. xii.).
‘Rightly to understand the question here, the first thing to be considered is the substantial generation of the vital spirit—a compound of the inspired air with the most subtle portion of the blood. The vital spirit has, therefore, its source in the left ventricle of the heart, the lungs aiding most essentially in its production. It is a fine attenuated spirit, elaborated by the power of heat, of a crimson colour and fiery potency—the lucid vapour as it were of the blood, substantially composed of water, air, and fire; for it is engendered, as said, by the mingling of the inspired air with the more subtle portion of the blood which the right ventricle of the heart communicates to the left. This communication, however, does not take place through the septum, partition or midwall of the heart, as commonly believed, but by another admirable contrivance, the blood being transmitted from the pulmonary artery to the pulmonary vein, by a lengthened passage through the lungs, in the course of which it is elaborated and becomes of a crimson colour. Mingled with the inspired air in this passage, and freed from fuliginous vapours by the act of expiration, the mixture being now complete in every respect, and the blood become fit dwelling-place of the vital spirit, it is finally attracted by the diastole, and reaches the left ventricle of the heart.
‘Now that the communication and elaboration take place in the lungs in the manner described, we are assured by the conjunctions and communications of the pulmonary artery with the pulmonary vein. The great size of the pulmonary artery seems of itself to declare how the matter stands; for this vessel would neither have been of such a size as it is, nor would such a force of the purest blood have been sent through it to the lungs for their nutrition only; neither would the heart have supplied the lungs in such fashion, seeing as we do that the lungs in the fœtus are nourished from another source—those membranes or valves of the heart not coming into play until the hour of birth, as Galen teaches. The blood must consequently be poured in such large measure at the moment of birth from the heart to the lungs for another purpose than the nourishment of these organs. Moreover, it is not simply air, but air mingled with blood that is returned from the lungs to the heart by the pulmonary vein.
‘It is in the lungs, consequently, that the mixture [of the inspired air with the blood] takes place, and it is in the lungs also, not in the heart, that the crimson colour of the blood is acquired. There is not indeed capacity or room enough in the left ventricle of the heart for so great and important an elaboration, neither does it seem competent to produce the crimson colour. To conclude, the septum or middle partition of the heart, seeing that it is without vessels and special properties, is not fitted to permit and accomplish the communication and elaboration in question, although it may be that some transudation takes place through it. It is by a mechanism similar to that by which the transfusion from the vena portæ to the vena cava takes place in the liver, in respect of the blood, that the transfusion from the pulmonary artery to the pulmonary vein takes place in the lungs, in respect of the spirit.
‘The vital spirit (elaborated in the manner described) is at length transfused from the left ventricle of the heart to the arteries of the body at large, and in such a way that the more attenuated portion tends upwards, and undergoes further elaboration in the retiform plexus of vessels situated at the base of the brain, in which the vital begins to be changed into the animal spirit, reaching as it now does the proper seat of the rational soul. Here, still further sublimated and elaborated by the igneous power of the soul, the blood is distributed to those extremely minute vessels or capillary arteries composing the choroid plexus, which contain or are the seat of the soul itself. The arterial plexus penetrates even the most intimate part of the brain, its constituent vessels, interwoven in highly complex fashion, being distributed over the ventricles, and sent to the origins of the nerves which subserve the faculties of sensation and motion. Most wonderfully and delicately interwoven, these vessels, although spoken of as arteries, are really the terminations of arteries proceeding to the origins of nerves in the meninges. They are in truth a new kind of vessels; for, as in the transfusion from arteries to veins within the lungs we find a new kind of vessels proceeding from the arteries and veins, so, in the transfusion from arteries to nerves, is there a new kind of vessels produced from the arterial coats and the cerebral meninges.’ ‘Chr. Rest.’ p. 170.
There can be no question as to the fact that, in the above quotation, the passage of the blood from the right to the left side of the heart through the lungs by the pulmonary artery and vein, is proclaimed, and a farther transmission of its more subtle part at least from the left ventricle of the heart to the arteries of the body is indicated. After so much said, however, the account halts. There is no notice of any transfusion from the arteries to the veins of the body, and so of a return of the blood by their means to the right side of the heart—nor do we believe that anything of the kind was present to the mind of the writer. The truth is that Servetus was not thinking of a circulation of the blood in the sense in which we understand the term, but of a means of engendering the vital and animal spirits. ‘The blood,’ he says happily and well, ‘is not sent to the lungs in such large quantity for their nourishment only. As in the fœtus, so in the adult are they nourished from another quarter.’ To Servetus as to his age the liver was the fountain of the blood, and the venous system connected with it the channel by which materials for the growth and nourishment of the body were supplied. The heart again was the source of the heat of the body, and, with the concurrence of the lungs, the elaboratory of the vital spirits; the arterial system in connexion with it being the channel by which the spirit that gives life and special endowment to the bodily organs is distributed.
Though Servetus saw that the black blood which is attracted, as he says, by the diastole of the heart from the vena cava acquires the florid colour in its passage through the lungs, he never hints at the black blood of the systemic veins having been the florid blood of the arteries. We are not, however, to overlook his remark, though it is only by the way, of ‘the natural spirits being communicated from the arteries to the veins by their anastomoses.’ Servetus may consequently have had an intimation of the systemic circulation; but he did not think out his thought. He does not speak of an intermediate system of vessels between the arteries and veins of the body as of certain other corresponding vessels of the lungs; and when we find him making the arteries of the brain terminate in the nerves or meninges—the source of the nerves to the old physiologists, we can only conclude that he believed the arteries of the body to end in like manner in the several tissues to which they are distributed. From what he says further concerning the life of the fœtus in utero, we learn positively that Servetus had not divined the systemic circulation. ‘The embryo lives through the soul of the mother,’ says he, ‘it is as it were a part of the mother, the vital spirit being communicated to it by the umbilical arteries.’ Instead of afferent canals of the blood from the heart of the fœtus to the placenta of the mother, consequently, Servetus believed the umbilical arteries to be efferent channels of the vital spirit of the mother to the heart of the fœtus. He at the same time, doubtless, saw the umbilical veins as the channels by which material for its growth and nutrition was brought from the mother to be distributed by the venous system proceeding from the liver and vena cava, in conformity with the physiological views of his age. Servetus did not think of the fœtal heart save as the passive recipient of life. He never heard its rapid tick tack, nor dreamt of it any more than he did of the heart of the adult as the agent in the general distribution of the blood in a great circle from arteries to veins, from veins to arteries, unbroken in the embryo, but complicated when independent life is assumed by the necessary passage through the lungs.