13. Other animals have no hair internally, nor upon the bottom of their feet, though it is on the upper part. The hare alone has hair on the inside of its cheeks, and upon its feet, and the mysticetus[95] has no teeth in its mouth, but hairs, like hog's bristles. The hair, if it is cut off, increases below, but not above. Feathers do not grow either above or below, but fall out. The wing of the bee, if it is plucked off, does not grow again, nor that of any other creature which has an undivided wing; nor does the sting of the bee grow after it is plucked out, but the animal dies. [95] Balæna Mysticetus. CHAPTER XI. 1. There are membranes in all sanguineous animals. Membrane is like a dense thin skin, but it differs in kind, for it is neither divisible nor extensible. There is a membrane round every bone and every intestine, both in the greater and smaller animals; they are inconspicuous in small animals, owing to their thinness and small size. The principal membranes are two, which surround the brain, one round the bones of the head, and this is stronger and thicker than that round the brain itself; and after these, the membrane which surrounds the heart. A thin membrane does not unite after it has been cut asunder, and the bones, when deprived of their membranes, become inflamed. 2. The omentum is a membrane. All sanguineous animals have an omentum; in some it is fat, in others it contains no fat. In viviparous animals, with cutting teeth in both jaws, it has its origin and is suspended from the middle of the stomach, where it appears like a suture of this organ. In those that have not teeth in both jaws, it is suspended in the same way from the principal stomach. 3. The bladder also is membranous, but its character is different, for it is extensible. All animals have not a bladder, but all viviparous animals have this organ, and the tortoise alone of oviparous animals. When the bladder is cut it does not re-unite, except at the very origin of the urethra, or only very rarely, for it has happened sometimes. No moisture passes into the bladder of dead animals; but in living creatures there are dry compounds, from which are formed the stones that are found in persons labouring under this disease; sometimes they are of such a nature in the bladder as to differ in nothing from shells. This, then, is the nature of veins, sinews, and skins, and of muscle and membrane; and of hair, nails, claws, hoofs, horns, teeth and beaks, and of cartilage, bone, and their analogues. CHAPTER XII.