These membranes have more vessels than all the other divisions of the fibrous system; they are perforated by a great number of foramina for the passage of these vessels, most of which only pass through them, and afterwards go to the organs they cover. These foramina, each of which is larger than the branch it transmits, form also a character of the fibrous membranes, distinct from the serous, which are always folded up, and never open to allow the vascular system to penetrate their respective organs. The particular description however of the membranes of which we are treating, will be added to that of the organs they surround. I shall except the periosteum, whose description belongs to and may be made in a general way, whether because clothing the whole osseous system, we cannot consider it separately, or because, as I have said, it is the centre from which arise and to which go all the organs of the fibrous system, so that its functions relate still more to this system than to that of the bones. III. _Of the Periosteum. Of its Form._ This membrane surrounds all the bones. Hard, resisting, of a grey colour, it forms for them a covering which extends everywhere, except where the cartilages cover them. Its thickness is remarkable in infancy; it is thinner in proportion in the adult and becomes more firm and compact. The ancients described it as extending from one bone to another over the articulation, and thus forming a continuous sac for the whole skeleton. This idea is incorrect. At the junction of the bones, the periosteum intermixes with the ligaments which serve it as a means of communication, and it is in this way only that we can understand its continuity. The crown of the teeth is destitute of it, as well as all the osseous productions that grow upon the head of certain animals. The periosteum is feebly united to the bone in infancy; it can then be separated with great ease, especially on the middle part of the long bones. In the adult, as the calcareous substance gradually encrusts its most internal fibres, the adhesion becomes very strong; it is extreme in old age, in which this membrane is often reduced to a very delicate layer by the progress of ossification. The constant pressure exerted by the muscles in their contractions, can also have a little influence upon this adhesion. Various elongations pass from the periosteum to the bone. They are much more numerous at the extremities of the long bones and upon the short bones, than upon the middle of the long bones, or the broad ones; which may easily be conceived of, from the much greater number of foramina in one than the other part. These elongations accompany the vessels, line the canals which pierce through the bone, are lost in those which terminate in its substance, do not penetrate the medullary cavity, and confined to the osseous texture alone, establish, between it and the membrane from which they arise, immediate relations.