2d. The cavities of reception are those which serve to receive an organ, lodge and defend it; such are the fossæ of the bones of the cranium, those of the ossa ilii, &c. These cavities sometimes belong to the whole of the bone, the form of which is concave, as we see in the frontal bone, sometimes they are hollowed out upon an insulated part, like the maxillary depression of the inferior jaw; they are always destined for an essential part, for a gland, a viscus, &c. 3d. The cavities of slipping are in general found at the extremity of the long bones. They are grooves, more or less deep, in which the tendons glide to go to the place in which they are inserted. All are covered with a cartilage and terminated by a very strong ligamentary ring. Do the tendons by their friction form these cavities? This is the common opinion, but it does not appear to me more probable than the theory of muscular, vascular impressions, &c. These cavities ought to be then so much the deeper, in proportion as the muscles are the more exerted; they ought not to exist in subjects paralytic from their infancy; they ought not to exist in the cartilages of ossification in the fœtus, whose limbs have hardly ever moved; but the contrary of all this is constantly observed. Let us describe then all the different configurations of the bones, as a consequence of the laws of ossification, laws in obedience to which the osseous forms, all primitively determined, are made to develop. The size of the extremities of the long bones favours the existence of these different cavities, which cannot on this account injure the osseous solidity. 4th. The cavities of impression correspond with the eminences of the same name. I have spoken of them above. 5th. The cavities of transmission are especially destined for the vessels and the nerves. We find many of them on the head; they have sometimes the form of a groove, sometimes that of a tube and at others that of a slit, according to the thickness or breadth of the bones which these vessels or nerves traverse in order to go from one place to another. The periosteum lines them; they contain more or less cellular texture. The nerves and vessels they transmit are foreign to the bones. 6th. The cavities of nutrition, on the contrary, give passage to vessels which carry to the bones or the medullary organ the substances that repair them. They are of three sorts. The first form canals that are seen on the long bones exclusively, and go to the medullary cavity. Each bone has but one of these, situated always on its body, directed obliquely between the fibres of the compact texture, running sometimes from below upwards, sometimes from above down into the cavity of the bone, and thus forming a communication from without to within for the vessel of the medullary organ. This foramen serves particularly for the exhalation and nutrition of this organ, and nourishes the bone only secondarily.