This membrane receives, as we have said, the insertion of almost all the fibrous system, of the tendons, the ligaments and especially the aponeuroses. This insertion has no connexion with the bone in infancy; but ossification soon seizing upon the most internal laminæ, all the fibrous organs appear to be identified with the bone in the adult. I would observe that this arrangement coincides with the prodigious power of drawing that the muscles, having become more developed, often exert at this age, and which only spent upon the periosteum, as it would have been without its ossification, would not have found in it a sufficient resistance, whereas acting also upon the bone, it moves it without endangering its covering. The general organization, the properties and the life of the periosteum are the same as those of the fibrous system; I shall not treat of them. _Development of the Periosteum._ In the fœtus, this membrane is soft, spongy, containing much gelatinous fluid; it melts easily in water; its fibres are not distinct; they become so as we advance in age, and at the same time the softness diminishes and the resistance increases. The periosteum in old age has extreme tenacity; it resists ebullition almost as much as the ligaments; those who prepare skeletons know this very well. It tears in various places, because its fibres in contracting are detached from the bone; but what remains becomes with great difficulty gelatinous. _Functions of the Periosteum._ The periosteum defends the bones which it covers from the impression of the moveable parts that surround it, from that of the muscles, of the arteries, whose pulsation would wear them, as happens in certain aneurismal tumours near the sternum, the vertebræ, &c. It is a kind of parenchyma of nutrition in reserve, if I may so express myself, always ready to receive the phosphate of lime, when it cannot be carried upon the bone that has become diseased; hence natural and artificial necroses which never take place in the teeth, from the want of this membrane. These little bones have caries and various alterations, but not true necrosis. We cannot doubt that the internal laminæ of the periosteum are successively ossified, and thus contribute not a little to increase the bone in thickness, when its increase in length is finished. I would observe upon this subject, that not only it, but all the fibrous system, has a singular affinity with the phosphate of lime. Next to the cartilaginous system, it has the greatest tendency to be encrusted with it, no doubt because its kind of general vitality, of organic sensibility in particular, has much analogy with that of the bones. Where the tendons in sliding upon the bones experience great friction they become osseous. The dura-mater and the tunica albuginea are very often ossified; the sclerotica serves as a parenchyma for much earthy substance in birds, which in consequence have it extremely hard.