This vascular net-work has only organic sensibility and insensible organic contractility, which are necessary for its functions; and it is this which especially distinguishes it from the medullary system of the middle part of the long bones, whose animal sensibility is, as we shall see, very great. Irritate in a living animal the interior of a short or flat bone, or the extremity of a long one, no sign of animal sensibility is manifested. Sawing the cranium, the condyles of the femur and the head of the humerus is not painful. Injuries of this system when they are very great may produce necrosis of the bone, and the formation of a new osseous substance at the expense of the periosteum; but if a small portion only is affected, this phenomenon does not take place. I have many times perforated transversely with a gimblet the extremity of a long bone of an animal, and afterwards passed a red hot iron through the opening; the animal has always recovered without necrosis; the articulation has only remained swelled, and much injured in its motions, and some scales have come from it during the suppuration. IV. _Development._ The vascular net-work which forms this medullary system, exists in the cartilaginous state; but then, on the one hand, it does not admit the red portion of the blood, and on the other, the interstices of its meshes are found so filled with gelatine, that the cartilage appears homogeneous. At the period of ossification, the red blood penetrates on one side of these vessels, whilst on the other they become evident from the absorption of gelatine at the place of these cells, upon the internal surface of which they ramify.