When the bone has all its dimensions it still continues to be the seat of a very active nutrition; exhalation constantly brings to it gelatinous and calcareous substances which absorption afterwards takes up; so that it is continually composed and recomposed. The experiment with madder evidently proves this; if we feed an animal for some time upon this, the bones become red much more easily, in proportion as the animal is younger; so that by amputating a limb after some time, the bones have an appearance wholly different from what is natural to them; if, after this amputation, we discontinue the use of the madder for some time, and then amputate another limb, the bones will be found to have entirely resumed their natural colour; now we know that the calcareous substance is the vehicle of the colouring matter, since while the bones are only cartilaginous the madder has no effect upon them. The calcareous substance is then alternately furnished and taken away from the bones. Besides, the formation and resolution of exostoses, the softening and brittleness of the bones, the phenomena of the production of callus, &c. are they not an evident proof of the exhalation and absorption of this principle? It appears clearly that the urinary system is the way by which nature gets rid of the calcareous and even of the gelatinous substance. It would be curious to analyze accurately the urine of ricketty patients, and that of those affected with cancer; it is probable that the first of these substances predominates in the urine of the first, and the second in that of the others; I know of no positive experiments upon the subject. Can we, by giving to patients gelatine or the phosphate of lime, restore to their bones the suppleness or solidity which they have lost? No, because it is necessary not only to introduce these substances into the economy but also to restore to the bones their peculiar organic sensibility which they no longer have, and which, by placing these substances in relation to them, would enable them to appropriate these to their own nourishment. The blood might be loaded with earthy and gelatinous principles, and the bones would repel these principles, so long as their sensibility was not in relation with them. The double motion of nutrition continues always in the bones, as we advance in age; but its proportions change. The gelatine is constantly diminishing and the calcareous substance constantly increasing. Finally, in extreme old age this last predominates so much, that it would destroy their life, if general death did not take place before that of the bones. It is to this that must be attributed the greyish colour that these organs then take; hence also their constantly increasing weight; hence consequently the difficulty of the motions of the limbs, since at the same time that the force of the muscular powers is diminished by age, the osseous resistance which they have to overcome increases.