The bones are hard, also, in those animals[311] which do not grow fat; those of the ass are used by musicians for making flutes. Dolphins have bones, and not ordinary fish-bones; for they are viviparous. Serpents, on the other hand, have bones like those of fish. Among aquatic animals, the mollusks have no bones, but the body is surrounded with circles of flesh, as in the sæpia and the cuttle-fish, for instance; insects, also, are said to be equally destitute of bones. Among aquatic animals, those which are cartilaginous have marrow in the vertebral column; the sea-calf has cartilages, and no bones. The ears also, and the nostrils in all animals, when remarkably prominent, are made flexible by a remarkable provision of Nature, in order that they may not be broken. When cartilage is once broken, it will not unite; nor will bone, when cut, grow again, except in beasts of burden, between the hoof and the pastern. Man increases in height till his twenty-first year, after which he fills out; but it is more particularly when he first arrives at the age of puberty that he seems to have untied a sort of knot in his existence, and this especially when he has been overtaken by illness. CHAP. 88.—THE NERVE: ANIMALS WHICH HAVE NONE. The nerves[312] take their rise at the heart, and even surround it in the ox; they have the same nature and principle as the marrow. In all animals they are fastened to the lubricous surface of the bones, and so serve to fasten those knots in the body which are known as articulations or joints, sometimes lying between them, sometimes surrounding them, and sometimes running from one to another; in one place they are long and round, and in another broad, according as the necessity of each case may demand. When cut, they will not reunite, and if wounded, it is wonderful what excruciating pain they cause; though, if completely cut asunder, they are productive of none whatever. Some animals are destitute of nerves, fish, for instance, the bodies of which are united by arteries, though even these are not to be found in the mollusks. Wherever there are nerves found, it is the inner ones that contract the limb, and the outer ones that extend it.