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.
Upon this subject I will make a remark which appears to me to be very important; it is that the capillary system of the face is, more than that of all the other parts of the skin, exposed to be thus penetrated with blood. 1st. This is evident in the two cases of which I have just spoken, and in which the action of the heart is increased. 2d. In the passions, the skin remains the same in the other parts, whilst that of the face suddenly becomes pale or red. 3d. We know that physicians frequently examine the state of the facial capillary system, which is almost always affected by the state of the internal viscera, and is full of blood or empty, according as it is sympathetically influenced. 4th. In various kinds of asphyxia, in those especially produced by submersion, by the vapour of charcoal, by strangulation, &c. the face is uniformly of a violet colour from the passage of the black blood into its external capillary system, into which it is brought by the arteries. Oftentimes the neck and the upper part of the chest are also livid; but there is never a discoloration of the inferior parts. 5th. In many diseases, in which death takes place by a kind of asphyxia, because the lungs are the first interrupted, the dead bodies have a violet-coloured and swollen face; this may be easily observed by all who frequent dissecting rooms. There are a hundred subjects in which the head has this lividity, to one in which it is observed in the inferior parts. 6th. Most cases of apoplexy produce the same lividity of the face.
To what is this extreme susceptibility of the facial capillary system to admit the blood owing? Three things, I think, principally contribute to it. 1st. The course is already opened to this fluid, since the redness of the cheeks necessarily supposes its presence in them, it only increases in quantity; whereas when another part of the dermoid surface becomes red, all the blood which enters it is almost accidental. 2d. The anatomical arrangement of the capillary system is more favourable to this passage there than elsewhere; for it appears that the communications of this system with the arteries of the chorion are more free. What proves this is, that in injections the face is coloured with great ease. There is undoubtedly no anatomist who has not been struck with this phenomenon, especially in children, in whom if the coarse injections of our dissecting rooms succeed at all, the face becomes wholly black, whilst the fluid penetrates but very little into the other parts of the cutaneous system. 3d. It appears that there is a greater sensibility in the face; in fact the same irritant brings blood there, which does not make it flow to any other place. For example, a blow equal to a box on the ear does not redden the skin of the arm, whilst it suddenly inflames the cheeks.
The blood disappears from the facial capillary system as it enters it; in an instant the passions will successively produce there the bright red of a paroxysm of fever, the whiteness of syncope and all the intermediate shades. It is even the extreme ease with which this fluid penetrates this system, that renders the face well adapted to serve as a kind of picture, which the passions paint by turns with a thousand shades, that are effaced, altered, modified and return again according to the state of the mind.
I would observe upon this subject that the passions have in the face three means of expression; 1st, the capillary system, a means wholly involuntary, and which often betrays what we wish to conceal; 2d, the muscular motion, which, by contracting or expanding the features, expresses the melancholy or gay emotions, and to which belongs as effects, the various wrinkles of which we have spoken; 3d, the state of the eye, an organ, which, as Buffon has remarked, not only receives the sensations, but also expresses the passions. The two last means are to a certain extent voluntary; we can at least disguise them; whereas we cannot deceive by the first. The actor imitates anger, joy, &c. because we can give these passions by contracting the eye-brows, by dilating the face in laughing, &c. But it is the rouge of the actress that imitates modest chastity; it is by removing this rouge that she imitates the paleness of fear, horror, &c.
I will add another essential observation in respect to the facial capillary system; it is that it appears that its tendency to receive blood, disposes it to become the more frequent seat of many affections. We know, 1st, that erysipelas is much more frequent in this than in other parts; 2d, that the variolous pustules are remarkably conspicuous here; 3d, that many eruptions are more abundant here than elsewhere.
From all that we have just said, it is evident that it is necessary to distinguish two portions in the capillary system exterior to the chorion. 1st. One is constantly filled with the colouring substance of the skin, a substance which appears to be stagnant like that of the hair of the head, and that of the hair of the body, which is subjected only to the slow and insensible motion of composition and decomposition and which never exhibits that sudden increase or diminution of which we have just spoken. 2d. The second is constantly pervaded by many fluids which continually succeed each other there, and which constantly escape by transpiration, and which are replaced by the blood, that insinuates itself into this portion of the capillary system. These two portions are entirely independent, and have probably no kind of communication.
It appears that at the instant of death there remains a certain quantity of the white fluids in the second portion of the exterior capillary system; the following experiment, which I have frequently made, proves it; by plunging a portion of skin into boiling water, and leaving it there an instant, the epidermis is raised up, not as a whole as in a blister, but in an infinite number of small vesicles which are formed suddenly on its surface, and which contain a serous fluid, that escapes as soon as we open these vesicles.