The blood does not penetrate it in an ordinary state, but a thousand causes can at every instant fill it with this fluid. Rub the skin briskly, and it reddens in a moment. If an irritant is applied to it, whether it acts mechanically like nettles, the appendices of which penetrate the epidermis, or exerts a chemical action, like the frictions with ammonia, or the action of fire when a portion of skin is held too near it, instantly the sensibility of this vascular net-work is raised; it invites into it the blood which it formerly repelled; every part of a surface reddens in proportion to the irritation. If passion acts powerfully upon the cheeks, immediately a sudden redness is evident in them. All rubefacients exhibit moreover a proof of the great tendency which the sensibility of the superficial capillary system of the dermis has to place itself, if it be ever so little excited, in relation with the blood which in the ordinary state is foreign to it. Vesicatories depend upon the same principle. Their first effect is to fill with blood the cutaneous capillary system, where they are applied, to produce in it a sudden erysipelas, and then to occasion a copious serous exhalation under the raised epidermis. They effect in a few hours what most cases of erysipelas do in many days; for we know that most of them terminate by vesicles which are raised above the skin. In burning, carried sufficiently far to be more than a rubefacient, and yet not so as to produce the horny hardening, there is also a sudden increase of exhalation under the raised epidermis. In general the production of every cutaneous bladder is always preceded by an inflammation of the external surface of the skin. This phenomenon is not exclusively confined to this system. We have seen the serous, as soon as it is laid bare and irritated considerably, redden in a short time by the passage of the blood into its exhalants; which constitutes an inflammation to which often succeeds a copious exhalation of milky or other kind of serum. This exhalation does not remain upon the surface, and does not form vesicles there, because it has no epidermis; this is the only difference between these phenomena, which at first view do not appear to be the same in the serous and cutaneous systems. It is not only the irritation of the cutaneous organ which makes the blood pass into the external capillary system. Whenever the heart is powerfully excited and it accelerates the course of this fluid, it always tends to go into it; this is what is evidently seen, 1st, after violent running; 2d, in the hot period of a paroxysm of fever.