[126] Harvey: On the Motion, etc., IV, Syd. 30, l. 14-18 and 30, l. 31 to 31, l. 4; Op. Omn. 32, l. 8-10 and 32, l. 22-30. [127] Aristotle: On the Parts of Animals, 666_a_, 8-13. [128] _Principium._ [129] Harvey: On the Motion, etc., XVI, Syd. 74, l. 4-15; Op. Omn. 75, l. 9-19. [130] It would be natural to conjecture that this Aristotelian slighting of spirits derived from the air, taken in connection with Aristotle's exaltation of the vital innate heat, may have had much weight with Harvey, who, although he used the word "spirits" freely, insisted that the blood and the spirits are one. But in this matter the Aristotelian precedent cannot have had the same force for Harvey that it would have for us, because he believed Aristotle to be the author of two treatises in which the spirits are expressly treated, not only as entities, but as entities of great physiological importance, though their relations with the outer air are neglected in one of the treatises and quite obscurely dealt with in the other. (See Harvey: On the Motion, etc., IV, Syd. 29, l. 16-25; Op. Omn. 31, l. 8-16. Do., VI, Syd. 38, l. 7-12; Op. Omn. 40, l. 2-5.) Indeed, Harvey in one of his references to Aristotle directly affirmed that the philosopher had believed in "motor spirits" within the animal body. (See Harvey: On the Motion, etc., XVII, Syd. 81, l. 8-12; Op. Omn. 82, l. 31 to 83, l. 3. Compare [Pseudo-] Aristotle: On Spirits, 485_a_, 5-8; and the Latin translation of the same "by an unknown interpreter," 249_b_, 13-18.) The two treatises in question are entitled, respectively, "On the Motion of Animals" and "On Spirits," and have been attributed to Aristotle and habitually printed among his works, both before and since the time of Harvey. Modern criticism, however, has made it clear that neither treatise is a genuine work of Aristotle. It is especially plain that the treatise "On Spirits" is by another hand and of another school; among other reasons, because the author declares the skin to be supplied with blood by the veins, and with spirits by accompanying vessels which he calls "arteries." In this treatise the maintenance of the spirits by respiration is discussed, but left uncertain (483_b_, 15-19). It is but fair to the criticism of Harvey's time to note that, glaringly at variance as the undoubted works of Aristotle are with the treatise "On Spirits," the latter was pronounced genuine, in 1839, by so eminent a scholar as É. Littré, in his "Œuvres d'Hippocrate," etc., Vol. I, 203. Reasons why the treatises in question are not by Aristotle, are given at length in the essay in Latin by V. Rose, entitled "De Aristotelis Librorum Ordine et Auctoritate Commentatio," Berlin, 1854, 162-171, and at the end of 174. [131] Compare the Iliad, Book XXI, 441. [132] ἐν τούτῳ γὰρ ἡ φύσις ἐμπεπύρευκεν ἀυτήν. Aristotle: On Respiration, 474_b_, 10-13. [133] Aristotle: On Youth and Old Age, etc., 469_a_, 11-12 and 17-20. [134] Plato: Timæus, 70_a_-_b_ and 77_d_-_e_.