The serous surface taken as a whole, compared with the cutaneous surface, is also evidently superior to it in extent; so that in this respect, the quantity of albuminous fluids constantly exhaled within, appears to be much more considerable than that of the fluid which is incessantly thrown off by insensible transpiration; I say in this respect, for different circumstances, by increasing the action of the cutaneous organ, can re-establish the equilibrium in the exhalation of these two fluids, one of which re-enters by absorption into the circulation, and the other is wholly excrementitious. I do not know even if the pulmonary and cutaneous exhalations united are not less than those which take place upon the serous surfaces. Every serous membrane represents a sac without an opening, spread upon the respective organs that it embraces, and which are sometimes very numerous, as in the case of the peritoneum, sometimes single, as in the case of the pericardium, covering these organs so that they are not contained in its cavity, and so that if it was possible to dissect them from their surface, we should have this cavity whole. This sac has in this respect the same arrangement as those night caps, which are folded within themselves; a trifling comparison, but which gives an accurate idea of this sort of membranes. From this general arrangement, it is easy to understand that the serous membranes are never opened to permit the vessels and nerves to penetrate the respective organs to which they go or from which they come off, but that they always wind round them and accompany them to the organ, and thus form for them a sheath which prevents them from being contained in their cavities; this removes the danger of infiltration of serum which lubricates them, an infiltration which would take place through the neighbouring cellular texture, especially if they were dropsical; if, as in the fibrous membranes, they were pierced with foramina for the passage of these vessels and nerves. This arrangement, exclusively remarkable in the membranes of which we are treating, and in the synovial ones, is evident at the entrance of the vessels of the lungs, the spleen, the intestines, the stomach, the testicles, &c. We see it very well in the arachnoides, a membrane essentially serous, as I have demonstrated elsewhere. From the general idea that we have given of these membranes, it is also easy to understand how almost all are composed of two distinct parts, though continuous, and embracing, the one the internal surface of the cavity where they are found, the other the organs of this cavity; thus there is a costal and pulmonary pleura, a cranial and cerebral arachnoides, one portion of peritoneum spread upon the gastric organs, and another upon the abdominal parietes, a free portion of the pericardium, and one adhering to the heart. The same arrangement exists in the testicles, &c.