The evacuation of the blood also establishes differences, though less, in the capillary system. Bleeding is of two kinds; one lessens the blood of the circulation of the great trunks; and then it is sometimes red, as in arteriotomy; but most often it is the black, that is drawn off; the other takes blood from the capillary circulation; this is done by leeches, cupping, &c. Each produces a different change in the course of the blood. Physicians formerly were desirous of knowing from which vein they ought to bleed. I think it is much more important to know when we should by bleeding, act upon the general circulation, and when upon the capillary. In many local congestions, I do not think that you can diminish the quantity of blood in a part of the capillary system, by diminishing the mass of this fluid in the great trunks; you might take a quarter at least of the blood that there then was in the economy, if the part is irritated, the blood will still flow as much to this part. On the contrary, you may double by transfusion, the mass of this fluid in an animal, local inflammations will not arise, because there must be a preliminary irritation before the blood flows towards, and enters a particular part of the capillary system.
The fluids differing from the blood which circulate in the capillary system, 1st. are evidently like it beyond the influence of the heart. 2d. The influence of the tonic powers presides over their motions. 3d. They are consequently subject to irregular oscillations, according as the capillaries are differently affected.
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.
The blood becomes more or less black in the general capillary system. If you have observed bleedings, you have undoubtedly seen in diseases innumerable varieties in the colour of the blood that comes from the vein. Has this fluid a different blackness in each part of the capillary system? It has appeared to me that the difference is not very great in this respect. I have frequently had occasion to open the renal, saphena, jugular veins, &c. the blood has appeared to me to be everywhere of nearly the same colour. I wished to see if the blood returning from an inflamed part was more or less black; I made then in the hind leg of a dog a number of wounds near each other, and left them open to the air. At the end of three days, when the inflammation appeared to be greatest, I opened high up on the diseased and the sound limb, the saphena and the crural veins, in order to examine their blood comparatively; I could discover no sensible difference. I bled a man who had a whitlow with an inflammatory swelling of the whole hand, and the inferior part of the fore arm; the blood appeared of the same colour as usual. Yet, as the veins bring also the blood of parts not inflamed, more minute researches must be made.
An object which deserves to be determined with precision, is this, viz. the cases in which, in general diseases, there is an alteration in the deep colour of the blood, and the symptoms which correspond with these alterations. At present we only know that it is more deep coloured in some cases and less so in others.