To the bony system belong the cartilages, which are formed of what might be termed tissue in a state of transition between fibrous and bony substance. This tissue, homogeneous in true cartilage, mingled with fibrous substance in the fibrocartilaginous parts, is elastic and flexible, and in colour either yellowish or pearl-white. The cartilages unite the bones at those points where, as in the chest, the bony frame must yield to expansive movements; they furnish a flexible skeleton to certain organs, as the external ear, for example, the nose, the eyelids, and the larynx; and lastly, they play an important part in the joints.
No part of the organism shows more clearly than the bony system the care which nature has taken to provide during infancy the gifts of which she is so prodigal in maturity, and which she withdraws little by little in old age. During infancy, when protected by maternal care and when the growth should be rapid, the gelatine predominates in the bones; they are flexible and have only a resistant power proportionate to the movements and efforts of infancy; it is the branch full of sap, but of which the woody portion is not yet developed. In youth, as the muscular power augments, the bones gradually become more solid; the extremities, at first cartilaginous, become ossified; the epiphyses unite with the body of the bone; and the articulating cartilages gain more consistence. In the adult at last the bones are complete. They are able to resist the muscular efforts of maturity, and perform their functions perfectly, like all other parts of the organism which are fully developed. But age approaches, the strength decreases, nutrition falls off the bones become more solid, their power of resistance lessens, the medullary canal enlarges, the proportion of calcareous salts in the osseous substance augments, the bones are harder but they are also more brittle. Thus, as each phenomenon of life is linked with every other, we find the bones of a child quickly and easily restored if broken; in the adult the process may be longer, but is generally easy and complete; in the old man the reunion of the fragments takes place slowly, and sometimes cannot be effected at all. The delicate twig, which afterwards became a vigorous branch, is now dry and destined to speedy decomposition.