(_H_) “All hearts contain cavities, but, in those of very small animals, the largest [cavity] is hardly visible, those of middling size have another, and the biggest all three. (_I_) “The point of the heart is directed forwards, as was mentioned at first; the largest cavity to the right and upper side of it, the smallest to the left, and the middle-sized one between these; both of these are much smaller than the largest. (_K_) “They are all connected by passages (συντέτρηνται) with the lung, but, on account of the smallness of the canals, this is obscure except in one. (_L_) “The great vein proceeds from the largest cavity which lies upwards and to the right; next through the hollow middle part (διὰ τοῦ κοίλου τοῦ μέσου) it becomes vein again, this cavity being a part of the vein in which the blood stagnates. (_M_) “The aorta [proceeds from] the middle [cavity], but not in the same way, for it is connected [with the middle cavity] by a much more narrow tube (σύριγγα). (_N_) “The [great] vein extends through the heart, towards the aorta from the heart. (_O_) “The great vein is membranous like skin, the aorta narrower than it and very tendinous, and as it extends towards the head and the lower parts it becomes narrow and altogether tendinous. (_P_) “In the first place, a part of the great vein extends upwards from the heart towards the lung and the attachment of the aorta, the vein being large and undivided. It divides into two parts, the one to the lung, the other to the spine and the lowest vertebra of the neck. (_Q_) “The vein which extends to the lung first divides into two parts for the two halves of it and then extends alongside each tube (σύριγγα) and each passage (τρῆμα), the larger beside the larger and the smaller beside the smaller, so that no part [of the lung] can be found from which a passage (τρῆμα) and a vein are absent. The terminations are invisible on account of their minuteness, but the whole lung appears full of blood. The canals from the vein lie above the tubes given off from the windpipe.” The key to the whole of the foregoing description of the heart lies in the passages (_G_) and (_L_). They prove that Aristotle, like Galen, five hundred years afterwards, and like the great majority of the old Greek anatomists, did not reckon what we call the right auricle as a constituent of the heart at all, but as a hollow part, or dilatation, of the “great vein.” Aristotle is careful to state that his observations were conducted on suffocated animals; and if any one will lay open the thorax of a dog or a rabbit, which has been killed with chloroform, in such a manner as to avoid wounding any important vessel, he will at once see why Aristotle adopted this view.