In the genuine works of Aristotle this Hippocratic doctrine does not reappear, though it is fairly certain that Hippocratic treatises which contain it were written before Aristotle's time. We have seen that the entrance of air into the heart, to cool the same, is an important feature of the Aristotelian physiology. Beyond the Aristotelian heart, however, we cannot trace the air which enters it. Yet we find "_pneuma_," "spirits," referred to by Aristotle, not seldom obscurely or in very general terms, as doing service, sometimes momentous service, in the physiology of generation and in certain workings within the bodies of full-grown creatures. In disease also spirits may play a very important part. These Aristotelian spirits, however, when their origin can be traced at all, are either innate or appear to be vapor produced within the body itself by heat or by disease. They do not appear to be recruited from the outer air which has penetrated the lungs and heart, that air seeming to complete its function within the lungs or within the heart itself by sustaining the native heat which is the great instrument of the soul, and in which the very soul itself is fired.[48] Physicians of Aristotle's time, however, revived and handed on the doctrine that not only blood but a derivative of the air is distributed to the body at large through the vessels. After the distinction between veins and arteries had been clearly made and the latter had received their present name, a striking modification of this doctrine of the spirits was adopted and pressed by the Greek physician Erasistratus, about 300 B.C., not many years after the death of Aristotle. This modified doctrine separated the paths taken within the vessels by the blood and the spirits derived from the air, and declared the transmission of the necessary blood to the body at large to be by the veins only, that of the necessary spirits, styled "vital," to be by the arteries only. More than four hundred and fifty years later Galen shattered this doctrine and incorporated the vital spirits in the arteries with the blood, which he proved by epoch-making experiments to be normally present in the arteries, he, however, clearly recognizing differences between the cruder blood in the veins and the spirituous blood in the arteries. The tissues, therefore, still received vital spirits by way of the arteries, according to Galen, but not spirits in their pure gaseous Erasistratean state.[49] Now let Galen tell us more in his own words:--