I would observe that in this respect it is necessary to distinguish acute from chronic affections. In the chronic inflammations, for example, of the pleura, of the peritoneum, &c. the redness continues after death, because the blood is combined, as it were, with the organ; it makes a part of it, as it makes a part of the muscles in a natural state. So the chronic affections of the skin, of the mucous surfaces, retain after death nearly the same blood, that they had during life; whereas in acute affections, the blood retained for a time by irritation, escapes when life has ceased, upon which this irritation depended. These principles can be applied to many diseases; I repeat it, they are of great importance in examining bodies. The neglect of them has often led me into an error, upon the degree and even the existence of acute inflammations, of which the organs that I examined had been the seat. V. _How, notwithstanding the general communication of the Capillary System, the Blood and the Fluids differing from it, remain separate._ Since in the dead body, and consequently during life, there is in the capillary system no organic obstacle to the communication of the fluids through its small branches; since the general net-work that these vessels form is everywhere free, how does it happen that the blood does not pass into the part destined to the white fluids? how is it that these do not enter where the blood is to circulate? Why does not this fluid go out by the exhalants and the excretories, since these tubes communicate with the arteries by the anastomoses of the capillary system? This depends wholly upon the relation which exists between the organic sensibility of each part of the capillary system, and the fluid that it contains. That which carries the blood, finds in all the other fluids irritants that make it contract at their approach; and reciprocally, where the other fluids belong, the blood would be a foreign fluid. Why does the trachea admit air, and reject every other fluid? Why do the lacteals choose only chyle from the contents of the intestines? Why does the skin absorb certain substances, and repel others, &c.? It all depends upon this, that each part, each portion of an organ, every organic particle, has its own sensibility, which is in relation only with one substance, and repels others.