‘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.