COMPOUND MEMBRANES. 131
organic laws, as immediate and direct as the adhesions contracted by the periosteum to the bone it covers, and to which it was not, in infancy, but very feebly attached.
205. Besides, the intimate connexion of the serous and fibrous membranes, is often essential to the functions of the part. Without it the synovial membrane plaited and rubbed in the violent motions of the joints, would soon be diseased, and would retard these motions. In general we do not find close connexions, and consequently fibro-serous membranes, except in all the organs which are not susceptible of great dilatation, as in the brain, testicle, etc., but where the organ is subject to obvious varieties of volume, as in the stomach, bladder, womb, etc., they would have hindered the different displacements, which the serous membranes must experience, as has been said, in adjusting itself to these varieties; this membrane is also every where loosely fixed by means of the cellular tissue.
§ 2. Sero-mucous membranes.
206. But few sero-mucous membranes exist in the animal economy; when these two simple membranes concur for the production of the same organ, they are almost always separated by an intermediate, commonly muscular, layer, as in the whole intestinal canal, the bladder, etc. The vesicula fellis however exhibits, at its lower part, the example of an immediate union. But in general the adhesion is not so intimate,
