Now, since it is when it has sense perception that an animal must first take food and receive growth, and in all cases food in its ultimate form is, in sanguineous animals, the natural substance blood, or, in bloodless animals, that which is analogous to this; and since the veins are the place of the blood, while the origin of these is the heart this is clear from the dissections it is manifest that, when the external nutriment enters the parts fitted for its reception, the exhalation enters into the veins, and there, undergoing a change, is converted into blood, and makes its way to their source.