And that milk is produced from blood by a change, is already clear; yet we may learn it from the flocks and herds. For these animals, in the time of the year which we call spring, when the air has more humidity, and the grass and meadows are juicy and moist, are first filled with blood, as is shown by the distension of the veins of the swollen vessels; and from the blood the milk flows more copiously. But in summer, again, the blood being burnt and dried up by the heat, prevents the change, and so they have less milk.