Ought we then to be surprised, if all the serous surfaces are not equally subject to the same diseases; if inflammation attacks them with such different degrees of violence; if it takes place ten times upon the pleura to once that it appears upon the arachnoides; if in the pericardium, the tunica vaginalis and the peritoneum, it does not exhibit the same symptoms; if dropsies vary also wonderfully in each; if the slow inflammations attack them differently, &c.? The pericardium is subject to an affection which I have seen upon no other serous surface, and which is yet extremely frequent upon this; I refer to the white layers, more or less broad, that are formed on its internal surface, which would be thought at first view to belong to its texture, which can however be raised from it leaving it sound. I do not know whence these layers come; do they correspond to the false membranes of the pleura? Neither should we be surprised at what has been said of the varieties which the same membrane exhibits in its diseases. Frequently the whole of the peritoneum is diseased, and the omentum remains sound and vice versa. The layers of which I have just spoken are seen upon the cardiac portion, and not upon the free portion of the pericardium. Observe however that all the diseases of this system have a common character which is evidently derived from the analogy of organization. This and the synovial are the only ones in which large serous collections take place, in which slow and tubercular inflammations are formed. The most of their modes of adhesion belong only to the serous system. Inflammation has a peculiar and distinctive character in it, of which all the serous membranes partake with some modifications. The inflammation of the meninges had been classed among the serous phlegmasias, from the analogy of the symptoms, before I had demonstrated that the arachnoides, one of these meninges, belongs essentially to the serous system. It is on account of this membrane, and not on account of the dura-mater which is of a fibrous nature, that phrenitis should be referred to the diaphanous membranes. ARTICLE THIRD. PROPERTIES OF THE SEROUS SYSTEM. I. _Properties of Texture. Extensibility._