This piece of incorrect physiology may well have received support from the fact that the pulmonary semilunar valve is commonly found to be not quite competent when the dead and dissected pulmonary artery of the bullock is distended with water--an observation which the ancient author intimates that he has made,[32] though he does not specify the creature dissected. Nearly five hundred years after the death of Aristotle, the analogy between life and flame was discussed, formally and at some length, by Galen. He knew his Aristotle well, and agreed with him as to the importance of respiratory cooling for protracting the indispensable heat of animals.[33] But we find Galen dealing with the uses of respiration in a less simple way than Aristotle. In a polemical treatise Galen debates the question whether "the breath drawn in in respiration" actually enters the heart, or whether it cools it without entering it. He says:-- "It is possible that the whole is breathed out again, as was believed by most physicians and philosophers, and those the keenest, who say that the heart, while it craves to be cooled, is in need not of the substance, but of the quality[34] of the breath, and that the use of respiration is indicated by the part.... I have shown in my treatise on the use of respiration that either an absolutely minute quantity, or none at all, of the substance of the air, is taken into the heart."[35] It is clear, however, that Galen, when delivering himself of the foregoing, was a trifle carried away by the ardor of contention; for in the very treatise to which he refers us, as well as elsewhere, he not only dilates upon the cooling effects of breathing, but admits the entrance of air into the heart for a definite physiological purpose. This purpose, however, which we shall study later, is not cooling and is counted of secondary importance by Galen. Nevertheless, he goes so far as to say this:-- "That some portion of the air is drawn into the heart in its diastole and fills the vacuum which is produced, is sufficiently shown by the very magnitude of the dilation."[36] In his treatise "On the Use of the Parts of the Human Body" Galen takes a more judicial tone in the following brief, calm summary:-- "The use of the respiration of animals arises from the heart, as has been shown. The heart itself needs in some sort the substance of the air; but, first and foremost, it craves to be cooled, because it boils with heat. The heart is cooled by the cool quality of inspiration; but expiration also cools, by pouring out that which seethes within the heart and is, in a way, burned up and sooty."[37] Thus do we see the modern products of respiration foreshadowed.