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,