Hence we see that the callus is cellular and vascular in the first period; that in the second it contains cellular texture and vessels, with gelatine; that in the third, it has cellular texture, vessels, and gelatine, with calcareous substance. It has not the regular forms of the sound bone, because the parenchyma of cicatrization arising irregularly upon the osseous surfaces, the exhalation and absorption of gelatine cannot be made in a precise and regular manner. The callus is so much the larger in proportion to the separation of the ends of the bone, because the fleshy granulations having had more space to go over in order to meet, are more extensive, and consequently have absorbed more nutritive substance. If the constant motion of the fractured ends prevents on each side the granulations, or what is the same thing, the two parenchymas of cicatrization from uniting, then, notwithstanding the exhalation of the nutritive substances in each of them, the bone does not unite, and hence the preternatural articulations. Callus is formed with difficulty when the ends divided and laid bare, suppurate with the neighbouring parts, as happens in compound fractures, because the formation of pus expends the nutritive substances destined to repair the fracture. The further considerations upon this singular production belong to pathology. I have not exposed in this chapter the ideas of the ancients, who thought that the bones were formed by the hardening of an osseous juice, the existence of which there is nothing to demonstrate; nor those of Haller, who imagined that the heart hollowed out arterial channels in the osseous substance by its own impulse, and hardened this substance by the pulsation of the arteries; nor those of Duhamel, who made every thing depend upon the periosteum; I refer to various works that have a thousand times refuted these opinions. Without refuting any one in particular, I would remark that they have one fundamental error, viz. that of considering osseous nutrition in an insulated manner, of not presenting it as a division of general nutrition, of admitting for its explanation reasonings only applicable to the bones, and which are not derived as consequences from those which serve to establish the nutrition of all the organs. Let us never lose sight of this essential principle, upon which rest all the phenomena of the economy, viz. that over a multitude of effects, a very small number of causes only presides. Let us mistrust every explanation which is partial and mutilated, which circumscribes the resources of nature according to the limits of our weak understandings. IV. _Peculiar Phenomena of the Development of the Teeth._