1st. In exhalation, there is no intermediate organ between the arteries and the exhalants; a capillary net-work alone separates them; whilst on the contrary there is always an intermediate organ between the excretories and the arteries; it is in this organ that the capillaries are found, in which the second begin and the first terminate. 2d. The organized machines which elaborate the secreted fluids are then much more complicated than those which separate the exhaled fluids. Thus the bile, the urine, the saliva, &c. differ on the one hand essentially from the blood, and are on the other much compounded; whilst the serum, &c. closely resembles some parts of the blood, and is but slightly compounded, containing but few elements. This double distinctive character of the two kinds of fluids appears to me to be very striking. 3d. The exhaled fluids are poured out by an infinity of small tubes separate from each other; the secreted fluids, on the contrary, are collected in one or more principal tubes that pour them on the surface where they terminate. 4th. The exhaled fluids re-enter in great part into the circulation, after having been thrown out; the secreted fluids, on the contrary, appear to be especially destined to be rejected. 5th. Many parts receive the first fluids; they are deposited upon the serous, mucous, synovial and cutaneous surfaces, in the cellular texture and even in all the organs of nutrition. The mucous and cutaneous surfaces, the first especially, are the only ones upon which the others are poured out. It follows from all these considerations, that the exhaled fluids, as the fat, the serum, the synovia, the marrow, &c. differ essentially from the secreted fluids, as the bile, the urine, the saliva, the mucous, prostate, spermatic, pancreatic fluids, &c. This difference appears to have struck a great number of authors; yet most of them have made use of the term secretion to express the separation of the exhaled fluids from the mass of blood. I think that there is much analogy between exhalation and secretion. In both, there is the capillary system, as I have said, between the vessel that brings and that which carries away; but the capillary system is certainly arranged very differently in a gland, from what it is in a serous surface; for example, wherever there is exhalation, there is certainly nothing but the capillary system; but where there is secretion, the secretory organ is too considerable not to admit of something more. Besides, by trusting to inspection, and without wishing to examine the intimate nature of the organs, it is evident that where there is secretion, there is a gland, and that this gland is wanting where there is exhalation. ARTICLE FIRST. GENERAL ARRANGEMENT OF THE EXHALANTS. I. _Origin, course, and termination._