I am disposed to think, on the contrary, that the words refer simply to the cavity of the pericardium. For a part of this cavity (_sinus transversus pericardii_) lies between the aorta, on the one hand, and the pulmonary vessels with the bifurcation of the trachea, on the other hand, and is much more conspicuous in some animals than in man. It is strictly correct, therefore, in Aristotle’s words, to say that where the heart and the windpipe are connected “it is hollow.” If he had meant to speak of one of the pulmonary veins, or of any of the cavities of the heart, he would have used the terms πόροι or κοιλίας which he always employs for these parts. According to Aristotle, then, the air taken into the lungs passes, from the final ramifications of the bronchial tubes into the corresponding branches of the pulmonary blood-vessels, not through openings, but by transudation, or, as we should nowadays say, diffusion, through the thin partitions formed by the applied coats of the two sets of canals. But the “pneuma” which thus reached the interior of the blood-vessels was not, in Aristotle’s opinion, exactly the same thing as the air. It was “ἀὴρ πολὺς ῥέων καὶ ἀθρόος” (“De Mundo,” iv. 9)—subtilised and condensed air; and it is hard to make out whether Aristotle considered it to possess the physical properties of an elastic fluid or those of a liquid. As he affirms that all the cavities of the heart contain blood (_F_), it is clear that he did not hold the erroneous view propounded in the next generation by Erasistratus. On the other hand, the fact that he supposes that the spermatic arteries do not contain blood but only an αἱματῶδης ὑγρόν (“Hist. Animalium,” iii. 1), shows that his notions respecting the contents of the arteries were vague. Nor does he seem to have known that the pulse is characteristic only of the arteries; and as he thought that the arteries end in solid fibrous bands, he naturally could not have entertained the faintest conception of the true motion of the blood. But, without attempting to read into Aristotle modern conceptions which never entered his mind, it is only just to observe that his view of what becomes of the air taken into the lungs is by no means worthy of contempt as a gross error. On the contrary, here, as in the case of his anatomy of the heart, what Aristotle asserts is true as far as it goes. Something does actually pass from the air contained in the lungs through the coats of the vessels into the blood, and thence to the heart; to wit, oxygen. And I think that it speaks very well for ancient Greek science that the investigator of so difficult a physiological problem as that of respiration, should have arrived at a conclusion, the statement of which, after the lapse of more than two thousand years, can be accepted as a thoroughly established scientific truth.