1. Concerning marrow, for this is one of the fluids which exist in some animals. All the natural fluids of the body are contained in vessels, as the blood in the veins, and the marrow in the bones, and others in membranes, skin, and cavities. The marrow is always full of blood in young animals; but when they grow older, in the adipose it becomes adipose, in fat animals fatty. There is not marrow in all the bones, but only in those that are hollow, and not even in some of these, for some of the bones of the lion have no marrow, others but little; wherefore some persons say the lion has no marrow at all, as was before observed. In the bones of swine there is very little marrow, in some none at all. CHAPTER XVI. 1. These fluids are nearly always co-existent with animal life; but milk and the spermatic fluid are produced afterwards. Of these the milk is always secreted in those animals in which it is present. The spermatic fluid is not secreted in all, but in some as in fishes are what are called melts. All animals having milk have it in the mammæ. All animals that are both internally and externally viviparous have mammæ, that is, all that have hair, as man, and the horse, the cetacea, as the dolphin, seal, and whale, for these also have mammæ and milk. 2. Those animals that are only externally viviparous, and oviparous animals, have neither mammæ nor milk, as fish, and birds. All milk has a watery serum, which is called whey, and a substantial part called curds; the thicker kinds of milk have the most curds. The milk of animals without cutting teeth in both jaws, coagulates, wherefore cheese is made from the milk of domestic animals. The milk of those with cutting teeth in both jaws does not coagulate, but resembles their adeps, and is thin and sweet; the milk of the camel is the thinnest of all, next is that of the horse; in the third place that of the ass. Cow's milk is thicker. 3. Under the influence of cold, milk does not coagulate, but becomes fluid; by heat it is coagulated, and becomes thick. There is no milk in any animal before it has conceived, or but rarely; but, as soon as it has conceived the milk is produced; the first and last milk are useless. Sometimes milk has come in animals not with young, from partaking of particular kinds of food; and even in aged females it has been produced so freely when sucked, as to afford nourishment for an infant. And the shepherds round Æta, when the shegoats will not endure the approach of the males, cut their udders violently against a thorn, so as to cause pain; at first, when milked, they produce bloody, and afterwards putrid milk, but at last their milk is as good as that of those which have young ones.