Sometimes however a derangement almost universal takes place in the capillary system, especially on the exterior; this takes place in sudden changes of the air. Though the vital laws preside essentially over the capillary circulation, yet the degree of pressure of the surrounding air can modify it to a certain point; we have a proof of this in cupping glasses, or in any other means that produce suddenly a vacuum upon a part of the body; then the fluids pressed in the neighbourhood by the external air, and not compressed on the contrary at the place of the cupping glass, raise up and distend considerably the skin. The sudden changes of the atmosphere produce upon the whole body, though in a less degree, the effect of a cupping glass. If the air is rarefied, the whole external capillary system is more full; even the sub-cutaneous veins swell; a very considerable part of the blood experiences then a derangement in its motion, between the two systems with red and black blood. The harmony, the correspondence of these two systems is disturbed; hence the uneasiness, the sense of weight, &c. of which we are instantly relieved by a sudden change of the atmosphere. 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.