The blood is not the only fluid that sometimes passes by the exhalants instead of the fluids that these small vessels naturally pour out. Who does not know how much the sweat differs? Sometimes water is almost alone transmitted by the skin; at other times the sweat is filled with many substances more or less heterogeneous; it is more or less salt; we know how very different at times is the odour of it. Observe the many substances that are thrown out upon the external surface by the exhalants, in the small pox, measles, scarlatina, &c. in herpetic affections, in different eruptions; compare the critical sweats with those that are natural, and you will see that the exhalants are, if I may so express myself, a common passage, which all the substances contained in the body can pass through, and which in fact they do pass through in certain cases, when, among the numerous modifications of which the cutaneous organic sensibility is susceptible, they find those that are in relation with them. Shall I speak of the serous exhalants? observe that the surfaces of the same name, according as they are affected, pour out many different fluids, a milky serum, and a thick substance that attaches itself to their surface in the form of a compact membrane, &c. If you have opened but little the bodies of those who have died of chronic peritonitis, you must have been astonished at the diversity of fluids then contained in the peritoneum. Grey, yellowish, fetid, without odour, thick, viscid, thin, &c. &c. these fluids are hardly twice the same. The serum appears to be always the general vehicle; but the substances that it contains, by the effect of the change that disease has produced in the vital forces of the membrane, are infinitely variable. Thus we shall see that the glands are a common way, through which pass, according to the manner in which they are affected, many substances which differ essentially from those that compose the secreted fluids in the natural state. IV. _Of the Preternatural Development of the Exhalants._