We know not the nature of most of these fluids, because they cannot be subjected to our experiments. They are those that enter the ligaments, the tendons, the aponeuroses, the hair, the cartilages, the fibro-cartilages, a part of the cutaneous, mucous, serous surfaces, &c. They communicate with the blood from which they arise, by the capillary systems, they afterwards move in their own systems. In most of the organs in which they exist alone, as in those called white, they are very slow in their motion, because the sensibility of these organs is obscure and dull. Thus different tumours, to the formation of which they contribute, have, as we shall see, almost always a chronic progress. There are often in the animal economy those tumours, that are commonly called lymphatic, though we are wholly ignorant of the fluids that form them. They are found especially in the neighbourhood of the articulations; but sometimes only the cartilages, the cellular texture, the bones, &c. are the seat of these white tumours; it is important to ascertain the characters that distinguish them from the tumours in which the blood especially enters. _Phenomena of the Alteration of the Fluids in the Capillary System._ We have just treated of the phenomena of the motion of the fluids in the general capillary system; let us now speak of the changes which they undergo there in their nature. The blood exhibits a remarkable phenomenon in the general capillary system; from red, which it was in the arteries, it becomes black. How does this take place? It evidently can happen only in two ways, viz. either by the addition or subtraction of some principles. Is it charged with carbon and hydrogen? Does it deposit only oxygen in the organs? Are these two causes united to give it its blackness? I think that it will always be difficult to decide upon these questions, which do not appear to me to be capable of any positive experiment. However, when we see the arterial blood furnish all the organs with the materials of their secretion, nutrition and exhalation, it is to be presumed that it leaves in these organs, rather than takes from them, the principle of its colour. Sometimes the red blood passes through the capillary system, without losing its colour; for example, when the blood has flowed for a long time black from a vein, we sometimes see it come out red, or nearly so, just before it ceases to flow. In opening the renal vein, I have two or three times made this observation, which has, I think, been noticed by some authors.