This cavity does not extend beyond the body of the bone; where the compact texture grows thinner, it disappears, and is replaced by a great quantity of the texture of the cells, which fills the extremity of the bone. Its form is cylindrical and its direction straight. It does not vary in its form, on account of its asperities or the external prominent lines of the body of the bone, which is only thicker in these places. Its parietes are much smoother in the middle, than at the extremities, where there are already many considerable cellular filaments thrown off. There are in many subjects, delicate, horizontal bony partitions, which interrupt almost entirely its continuity in this place, and appear to divide it into two or three very distinct parts. The medullary canal serves not only to lodge and defend the medullary organ, but also to give more resistance to the bone; for we know, that of two cylinders formed of an equal quantity of matter, one of which is hollow, and consequently has a greater diameter than the other which is full, the first will resist more than the second, because we can bend and break it with less facility. Full cylinders, equal in diameter to the long bones, would have prevented by their weight, the motions of the limbs; whilst other cylinders of the same weight as the present, but without any cavity, would give too small a surface for the insertion of the muscles. To unite small weight with a sufficient space in the middle of the long bones, is then a great advantage of the medullary canal. This canal disappears in the first periods of the formation of callus in fractures, because the whole medullary organ is occupied at this place by gelatine, and becomes cartilaginous; then this gelatine gradually re-absorbed, without being replaced, favours the development of a new cavity, and the communication is re-established between the superior and inferior parts of the canal. I have observed that, in the first age, and while the extremities of the bones are cartilaginous, the medullary canal is shorter in proportion than in the adult; it hardly forms at birth more than the middle third of the bone, the superior and inferior thirds being formed at first by the cartilaginous portion of each extremity, then a texture of cells intermediate between this portion and the canal; so that as we advance in age, its length becomes in proportion greater. II. _Of the Flat Bones._