It is probable that the insensible organic contractility, which is then brought into action by the contact of the air upon this membrane which contracts from its irritation, has an influence also upon this phenomenon; for this membrane evidently has this kind of contractility, as well as the corresponding sensibility. The animal sensibility is developed in it to an extreme degree in the natural state; the most acute pains are the result of the action of the saw upon it in amputation, of the introduction of a probe, of the injection of an irritating fluid into the medullary cavity, or of any other means which powerfully excite it. I do not speak of the pains of the bones in spina ventosa, syphilis, &c.; as the membrane is not then in a natural state, we cannot infer from them what kind of vital forces it is naturally endowed with. I have observed that the sensibility is greater, as we approach the centre of the bone with the probe when pushed into living animals. At the extremity of the medullary canal this sensibility is small; in the middle, the division of the bone is very painful. Whence arises this inequality of sensitive power, this decrease from the centre to the extremities? I know not. The medullary system evidently does not possess animal contractility and sensible organic contractility. It is evident from this view of the vital forces which animate this system, that the life is much more active in it than in the osseous system, that its vital phenomena are consequently more rapid, that they have not that chronic course which characterizes all the diseases of the bones and that they respond more promptly to the sympathetic excitements of other organs. I am persuaded that many of the uncertain pains which we usually refer to the bones in diseases, have their seat rather in the medullary system, in that of the middle of the long bones especially; observe in fact that most of these pains are fixed in the middle of the limbs, and that they are really in the direction of that system. The medullary system of the extremities of the long bones, and of the flat and short ones, certainly enjoys much more vital energy than the osseous texture itself; inflammation is much more easily developed in it, its effects are more promptly shown. Who does not know that caries is so much the more rapid, in proportion to the quantity of the texture of the cells that exists in the bones? It is not this texture, which by its nature, has an influence upon this phenomenon; but it is, because the more abundant it is, the more the medullary system predominates in it; now as this participates in all its affections, it imprints upon them a rapidity which they have not in the compact texture in which it does not exist. IV. _Development._