Chapter X: Part 10

Hence the internal face of the dermoid chorion exhibits three very distinct modifications. The first and last are seen to a small extent, whilst the second is almost general, with some differences however in the trunk, the extremities and the head. Besides, these modifications do not suppose a diversity of nature, but only of forms. Much separated and arranged in fibres in the first, the dermoid texture is compact and condensed a little in the second, and by this condensation renders the spaces less distinct. But there is a means of seeing them everywhere very well, where there is the least trace of them, and this is by maceration. This means also shows the dermoid texture best. In fact, when the skin has remained for some time in water, it softens, the fibres of its chorion separate, and their interstices become more distinct; then we see that the spaces exist not only on the internal surface, but that they extend into its texture which appears to be truly like a sieve in its whole thickness, so numerous are the spaces arising from the interlacing of the fibres.

These spaces do not terminate in culs-de-sac towards the external surface; they open upon this surface by many foramina which are very evident in a skin that has been macerated for a month or two, and which, in the ordinary state are almost imperceptible in some subjects, and very visible in others. Besides, in order to see them it is necessary to remove the epidermis; now as with the view of producing this effect immediately we commonly employ the action of boiling water or fire, the dermoid texture by this means acquires the horny hardening, and they become much less apparent, whereas maceration not only does not produce horny hardening of the skin, but it expands and dilates it, which renders these foramina very evident. In some parts of the skin and in certain subjects, we might then introduce the head of a pin into them; in others they are less evident. These foramina never pierce the dermis perpendicularly, all open obliquely to its surface; so that a perpendicular pressure tends to close them and bring their parietes in contact. I cannot compare their termination better than to that of the ureters in the bladder; hence why the hairs which go through them are never perpendicular, but oblique to the skin. We speak incorrectly when we say that the hairs are planted obliquely; their insertion in the bulb is perpendicular; it is in their passage through the chorion that they change direction.

Besides, these foramina are not vessels, but mere communications from the interior to the exterior through which pass the hairs, the exhalants, the absorbents, the blood-vessels and the nerves which go to the surface of the dermis; thus the subjacent spaces are only cells in which are contained the vessels of the glands and of the cellular texture. The dermoid texture should then be considered as a real net-work, as a kind of cellular texture, the cells of which very evident within, become less so on the exterior surface, with which all communicate to transmit to it different organs. The chorion is then the outline, the frame, if I may so say, of the cutaneous organ. It serves to lodge in its spaces, all the other parts which enter into the structure of this organ, and contributes to give them the form they are to have, but is wholly foreign to them.

What is the nature of this texture, which enters especially into the composition of the cutaneous chorion? I know not; but I think it has much analogy with the texture of the fibrous system; the following considerations support this analogy. 1st. On the heel, where the dermoid texture has the fibrous form of the irregular ligaments, it would be almost impossible to distinguish it from it, so uniform is the external appearance; it has the same resistance and density; the same sensation is experienced when it is cut with the bistoury. 2d. The dermoid texture becomes yellow and transparent like the fibrous by stewing. 3d. It melts gradually like it into gelatine. 4th. Like it, except the tendons however, it strongly resists maceration. 5th. Sometimes these two textures are identified; for example, the annular ligaments of the wrist evidently send elongations to the neighbouring dermoid texture. 6th. This texture can serve, like the fibrous, for the insertion of muscles; we see it in the face, where many of the fibres of the orbicularis of the lips and the eyelids, and almost all those of the eyebrows, find real tendons in the fibres of the dermoid texture. There is the same arrangement in the cutaneous palmar muscles.

All these considerations evidently establish many relations between the dermoid and fibrous textures. Yet they are far from being the same. To be convinced of this it is sufficient to observe how much their mode of sensibility differs, and how different also are their diseases; it seems at first as if there was no analogy between them in this double relation. Yet the line of demarcation is by no means as great as it appears to be. In fact the acute sensibility of the skin is not seated precisely in this white texture, which is interwoven so as to leave between its meshes the spaces of which we have spoken, and which we see especially on the surface adhering to this organ. The experiment mentioned in the article on the mucous system, and in which I irritated the cutaneous organ from within outwards, evidently proves it. It is the surface on which the papillæ are found that especially exhibits this vital property.

On the other hand morbid anatomy proves that the internal surface of the dermis, in which are especially found the texture and the spaces of which we have spoken, is entirely free from most cutaneous eruptions. This is no doubt true as it respects the small pox, the itch and many species of herpes; I have satisfied myself of it as to the vaccine vesicles, the miliary eruption, &c. &c. It is certain that in erysipelas, the external surface only of the chorion is coloured by the blood which enters the exhalants; thus the slightest pressure, causing the blood to flow back, produces a sudden whiteness which soon disappears by the return of the blood into the exhalants. It is this which forms the essential difference between simple erysipelas and phlegmon, in which not only the external face of the chorion, but its whole texture and the subjacent cellular one are inflamed. In measles and scarlatina, the redness is also very evidently superficial. These phenomena accord with those of injections; for if they succeed at all in children, the skin of the face and less frequently that of the other parts, becomes almost entirely black. Now this blackness is much more evident on the external than the internal surface of the skin, no doubt because more exhalants are found in the first than in the second, which the arterial trunks only traverse.

The preceding considerations evidently prove that the texture of the internal surface of the chorion, and even that of its interior, have a vital activity much less than that of the external surface; that this texture is disconnected with all the great phenomena which take place upon the skin, with those especially which relate to the sensations and the circulation; that it is in the papillæ that the first are seated and in the reticular body the second; and that it is almost passive in nearly all the periods of activity of this double portion of the dermis. Its functions, like those of the fibrous texture, suppose it to be almost always in this passive state; they are only to defend the body and to protect it from the action of external bodies. It is this which forms our real covering; thus its properties are well adapted to this use. Its resistance is extreme. It requires very considerable weight to tear very narrow strips of chorion, when it is suspended from them; drawn in various directions, these strips are broken also with much difficulty.

Yet this resistance is much less than when tannin is combined with the chorion. We know that when thus prepared, this portion of the skin affords the strongest strings we have in the arts. I know but two textures in the animal economy, which unite to such an extent suppleness and resistance; these are this and the fibrous texture; and this is a new character which approximates them. We have seen that it requires a very considerable weight to break a tendon, a strip of aponeurosis, or a ligament taken from a dead body. The muscular, nervous, arterial, venous, cellular textures, &c. yield infinitely more easily. If the dermoid texture had less extensibility, it might advantageously supply the place of the tendons, the ligaments, &c. in the structure of the body.

Since the chorion is foreign to almost all the sensitive and morbid phenomena of the skin, let us inquire then in what part of the dermis these phenomena are seated. These parts exist very evidently on the external surface; now we find on this surface, 1st, what is called the reticular body; 2d, the papillæ.

_Of the Reticular Body._

Most authors have considered the reticular body as a kind of layer applied to the external face of the skin between the chorion and the epidermis, pierced with an infinite number of openings through which the papillæ pass. I do not know how we can demonstrate this layer, which escapes according to the opinion of most of them, when the epidermis is detached. In order to see it I have employed a great many means, but no one has succeeded. 1st. Such is the adhesion of the epidermis to the skin, that in a sound state we can hardly separate them without injuring one or the other. Yet with the greatest precaution we see nothing mucous on the chorion when it is laid bare. 2d. A portion of skin cut longitudinally, especially from the foot where the epidermis is very thick, allows us to see very distinctly on the divided edge the boundaries of this and of the chorion; now nothing escapes from about the line which separates them. 3d. In ebullition in which the epidermis has been removed, nothing remains upon the internal surface, nor upon the chorion. 4th. Maceration and putrefaction, the latter especially, produce upon the chorion a kind of glutinous layer the instant the epidermis is removed. But this layer is entirely the product of decomposition. Nothing similar is met with in the ordinary state.

I believe, from all these considerations, that there is not a substance deposited by the vessels upon the surface of the chorion, extravasated, stagnant upon this surface, and representing there a layer in the sense in which Malpighi understood it. I believe that we ought to understand by the reticular body, a net-work of extremely fine vessels, whose trunks already very delicate, after having passed through the numerous pores with which the chorion is perforated, come and ramify upon its surface, and contain different kinds of fluids.

The existence of this vascular net-work is placed beyond a doubt by fine injections which change the colour of the skin entirely externally, without altering it much within. This is, as I have observed, the principal seat of the numerous eruptions most of which are really foreign to the cutaneous chorion.

We may then consider the reticular body as a general capillary system, surrounding the cutaneous organ, and forming with the papillæ a layer between the chorion and the epidermis. This system contains in most men, only white fluids. In negroes, these fluids are black. They have an intermediate tinge in the tawny nations. We know how much the shades vary in the human race. Hence the colouring of the skin resembles nearly that of the hairs, which evidently depends upon the substance existing in their capillary tubes; it is analogous to that of the marks at birth, that are commonly called nævi materni, and in which we never see a layer of fluids extravasated between the epidermis and the chorion.

Moreover, I think we know but little as yet concerning this substance, which fills a part of the external capillary system. It does not circulate in it, but appears to remain there till another replaces it. When we examine the skin of a negro, we see a black teint, and that is all. In maceration I have observed that this teint is sometimes removed with the epidermis, and that it sometimes remains adhering to the chorion. It is very evidently foreign to both, since both have the same colour in whites as in blacks. It is never reproduced, after it has been removed; for cicatrices are white in all people.

Is there in white people a white substance which, remaining in the external capillary system, corresponds to that of negroes, or does the colour of their skin depend only upon the epidermis and chorion? I have been tempted to believe that they also have a colouring substance, since the long-continued action of a powerful sun evidently blackens them. This circumstance has even made me believe that whiteness is natural to all men, and that there was but one primitive race which has degenerated according to different climates.

But in order to be convinced of the diversity of races, it is sufficient to observe, 1st, that the teint of the skin is but one of the characters which distinguish each race, and that many others are always united to it. The nature and form of the hair, the thickness of the lips and the nose, the width of the forehead, the degree of inclination of the facial angle, the whole appearance of the face, &c. are constant attributes which indicate a general modification in the organization, and not merely a difference of the dermoid system. 2d. White people become tawny in hot countries; but they never acquire the teint of the people of the country. 3d. Removed to cold countries in early age, or even born in them, the blacks always remain so; their shade hardly changes at all from generation to generation. 4th. Colour by no means follows temperature exactly; we see many varieties in the shades of people who live under the same degree of latitude, &c.

Every thing proves then that the colour of the skin is but an insulated attribute of the different human races, though it is that which is most striking to our senses, and that we should not attach to it a greater importance than to many others which are drawn from the stature, which is oftentimes very small, as in the Laplanders, from the broad and flat face, as in the Chinese, from the dimensions of the chest, of the pelvis, the extremities, &c. It is from the differences of the whole, and not from those of an insulated part, that the lines of demarcation should be made which separate the races. The European face and forms are in general the type with which we compare the exterior of the other nations. The ugliness or beauty of the human races are, in our way of considering it, measured by the distance which separates these races from ours. Such is in fact the force of habit with us, that we rarely judge in an absolute manner, and that every object which is much removed from those to which we are accustomed, is disagreeable to us and sometimes even disgusting.

Besides, the colouring matter of the cutaneous reticular body is more interesting to the naturalist than to the physician. What should particularly arrest the attention of the latter is the portion of the capillary system exterior to the skin in which the fluids circulate. In fact, besides the portion which is the seat of colour, there is evidently another that the white fluids constantly pervade, in which they are moved with more or less rapidity, and in which they continually succeed each other. It is from this portion that the exhalant pores arise which furnish the sweat; it is this vascular net-work which is the seat of erysipelas and of all the cutaneous eruptions that are foreign to the chorion.

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.

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.

_Papillæ._

We call by this name those small eminences that arise from the external surface of the chorion, and which, piercing the capillary net-work of which we have just spoken, become by their extremities contiguous to the epidermis. These eminences are very evident in the palm of the hand and the sole of the foot, where they are regularly arranged, in the form of small curved striæ in different directions. We see them through the epidermis, notwithstanding its thickness in these places. But they are seen especially when this has been in any way removed, as by maceration, ebullition, &c. If we cut longitudinally a portion of the chorion of the foot, with its epidermis adhering to it, we see between them along the divided edge, a line in the form of a curved thread, which arises from these small eminences placed at the side of each other.

In some other parts of the skin, we distinguish the papillæ in a very evident manner; but in a great number, the epidermis being removed, we see only a surface, slightly uneven from some small eminences, especially towards the orifices through which the hairs and the vessels pass, but we do not discover those regularly arranged eminences, the papillæ properly so called.

We must not mistake for them the numerous and very evident prominences, which render the skin of some subjects extremely rough. These prominences are formed by small cellular, vascular or nervous bunches, by sebaceous glands, &c. which are found near the small openings by which the chorion opens under the epidermis, and usually transmits the hairs. These bunches, lodged in the small oblique canals which are terminated by these openings, raise the external side of them and thus form this prominence. The following very curious experiment proves this arrangement; when the skin is macerated for two or three months, or even less, on the one hand these little bunches in which there is almost always a little fat, are changed into that white, thick, unctuous matter, analogous to spermaceti, into which fat kept a long time in water is always converted; and on the other, the foramina enlarging, as we have seen, and the skin changing into a kind of pulp, we can easily remove it all around these little prominences, and see that they are continued with the fat which fills the meshes of the subjacent chorion, and which is also changed into a hard matter.

Injections have evidently proved to me that there were vessels in these cellular bunches, and I have been convinced of it for some time past by the dissection of some subjects that died of scurvy, in whom the spots commenced by very small ecchymoses, similar as it were to flea-bites, and which occupied these little eminences. The petechiæ of adynamic fevers have a different appearance; but they belong also to an extravasation of blood in the cellular texture, occupying the small pores which open on the exterior of the chorion to transmit the vessels, the hairs, &c. The more prominent these eminences are, of which we have spoken, the more uneven is the skin. In general they are more frequent on the extremities and on the back, than on the anterior part of the trunk. In the extremities there are more of them in the direction of extension, than in that of flexion.

We attach the idea of a beautiful skin, to that in which these small tubercles are not found, and in which the chorion is united at its external surface. Women have commonly this last arrangement more evident than men. The epidermis which covers these eminences very often scales off at that place, especially from strong friction, which contributes still more to render the skin uneven, rough and harsh to the touch where they exist, which might induce a belief that they are formed by it, though it is always only accessory to them. Where it is very thick, as in the palm of the hand and the sole of the foot, it cannot be raised, and these small cutaneous tubercles are never seen. In the face where many vessels pass from within outwards, by the little pores of which we have spoken, we meet with hardly any more of them. The papillæ scattered among these eminences, are in general very slightly apparent in the places where they exist.

All anatomists attribute to these last a nervous structure; they regard them as the termination of all the nerves that go to the skin, and which, according to them, are expanded to form these, after having first left their external covering. Some even say that they have traced filaments even into the papillæ; I confess that I have never been able to do it. In the ordinary state, the density of the chorion and the extreme delicacy of the filaments, are evidently an obstacle to it. In the state of long continued maceration, in which the chorion becomes pulpy and in which we might consequently trace these filaments, were it ever possible, it cannot be done. I do not however deny the texture attributed to the papillæ. The acute sensibility of the skin seems even to suppose it; but it is only analogy and not demonstration, which establishes this anatomical fact; indeed all the other senses, whose organs are so sensible, have the portion of them which receives the impression of bodies continuous with a nerve.

_Action of different bodies upon the Dermoid Texture._

In most of the other textures, we have only considered this action in the dead body, because during life, these textures always removed from external bodies, cannot be influenced by them. Here we can regard it in a double relation, since the skin is incessantly in contact with almost all the bodies of nature.

_Action of Light._

Light evidently acts upon the dermis. Removed from its influence, men are blanched, if we may so say, like plants. Compare the inhabitant of a city, who is never exposed to the influence of the sun, with the peasant who constantly is, and you will see the difference. It appears that it is the light and not the heat which produces the effect of which I have already spoken; for individuals who live in a warm temperature, but removed from the solar light, become white like those of cold countries. Thus we know that some men who keep their chambers always very hot, are whiter than others who, living in a less hot atmosphere, are constantly exposed to the sun. We might remain forever in a bath of a temperature equal to the warmest seasons, and the skin would not blacken. Apartments for study which are warmed with stoves, and in which men remain as long as the labourer at his plough, are as warm as the atmosphere of summer, and yet the skin of those who occupy them never becomes darker. Besides an irresistible proof is that the clothing which does not prevent the action of caloric upon the skin, and which offers a barrier to the rays of light only, prevents the cutaneous colouring that takes place upon the parts which the light immediately strikes, as upon the hands, the face, &c.

I do not speak of the solar influence upon the vital forces of the skin, as in cases in which sun-strokes produce erysipelas, or as when light is employed medicinally to recall the life of a part; but it is only in relation to the dermoid texture that I consider its action.

_Action of Caloric._

The action of caloric upon the skin exhibits very different phenomena, according to the degree of it that it is applied.

1st. A warm atmosphere expands the dermoid texture, increases its action, and makes most of the fluids which form the residue of nutrition and digestion, pass off by the exhalants.

2d. When contracted by cold, this texture refuses to admit those fluids, which then go off principally by the urine.

3d. The insensible change from one to the other of these two states, does not disturb the functions. When this change is sudden, there are almost always alterations in different organs, because the fluids destined to pass out, cannot vary their direction as rapidly towards this or that organ, as the cutaneous excitement produced by the sudden changes from heat to cold.

4th. The skin resists a temperature much greater than that of the body; it opposes an insurmountable barrier to the external caloric, which tends to an equilibrium in animate as well as inanimate bodies. Thus whilst these last are penetrated with this fluid in a medium warmer than themselves, and soon acquire the temperature of this medium, living bodies remain at the same degree, how much greater soever the surrounding heat may be to their own. The curious experiments of the English physicians have placed this truth, as it respects man, beyond a doubt. It is unnecessary to give the detail of these well known experiments, in which the mercury was seen to descend in the thermometer, when the bulb was placed in the mouth and in which the skin became covered, in a heated room, with the aqueous vapours of the air, which the greater cold of the body condensed upon its surface. A slight attention to animals with cold blood, living in warm climates, proves the same thing. I will make one remarkable observation upon this point, it is, that most reptiles, whose temperature is much less than that of the mammalia and of birds, and who consequently are brought much nearer than them to the temperature of winter, cannot however support it. They become torpid and sleep in subterraneous places, the heat of which remains nearly uniform like that of cellars, and do not awake till the milder temperature of spring stimulates them.

5th. The skin, in very cold climates, seems to be on the other hand an obstacle which prevents the internal caloric from suddenly escaping and thus placing the body in equilibrium with the surrounding medium. This is evident in countries near the pole. Upon this subject an observation the reverse of the preceding can be made; it is that the cetaceous animals inhabit seas the temperature of which is most unlike their own. Whales are sought for especially in the latitudes of Greenland, Spitzbergen, &c. Why do fishes with warm blood delight in the frozen seas, whilst the amphibious animals with cold blood prefer the burning heat of the sun? I know not.

Let us observe that most of the internal organs when exposed in solutions of continuity, have not the faculty of preserving as well as the skin, a degree of independent temperature. They become cold or hot sooner than it as long as they remain healthy. The intestine brought out of the abdomen in the operation for hernia, a muscle laid bare, &c. &c. exhibit this phenomenon; thus in order to give them this faculty of having an independent temperature, nature inflames them, and they thus constantly preserve their heat, whatever may be that of the surrounding medium. The mucous surfaces next to the skin resist the surrounding temperature the most, as is seen in prolapsus of the rectum, in inversion of the anus, &c. This difference among the different systems is probably owing to that of their structure.

6th. When the action of caloric is carried to a very considerable extent, it begins to act upon the skin, and its effects are the more evident in proportion as it is the more intense. 1st. The slightest of these effects is to produce an evident redness, a kind of erysipelas; the caloric then acts like a simple rubefacient. 2d. The second is to redden the skin and then to produce vesicles on it. 3d. In the third there is a real horny hardening, a crisping of the fibres of the chorion which contract, like those of all the animal textures exposed to too strong a degree of heat. 4th. In the fourth and last effect, the dermoid texture is burnt, blackened and reduced to mere carbon. These different degrees of burning arise only from different degrees of caloric. I would observe that in the two first effects, this fluid acts upon the vital forces, and that these two effects cannot consequently take place except during life. The two last are exerted only upon the texture of the organ; thus they take place after death precisely as before. Cooks often employ the horny hardness, to give to the skin a hardness and brittleness necessary in some kinds of cooking.

7th. Cold carried to a great degree acts also upon the cutaneous organ, and produces different effects, according to its intensity. The first of these effects is very analogous to the first effect of a slight degree of caloric. It consists of a kind of local inflammation. The tip of the nose, the ears and the fingers, the cheeks, &c. become red from a slight degree of cold. I have not accurately observed the other effects between this and the last, which consists in a sudden privation of life. But there is this difference between the gangrene that then takes place, and that produced by a high degree of caloric, that the blackness is sudden in this last, whereas it takes place only as a consequence in the other. Observe in fact that there is in gangrene two things which physicians do not sufficiently distinguish, 1st, the mortification of the part; 2d, its putrefaction. The mortification is always antecedent; it is produced by a thousand different causes; sometimes by the ligature of an artery, as in aneurism; sometimes by that of a nerve; often by violent inflammation; sometimes by a contusion, attrition, a bruise, &c. Whenever a part is dead in the midst of those which live, whatever may be the cause of its death, it becomes putrid precisely like a dead body, every part of which life has left. Putrefaction takes place then even sooner, because on the one hand the natural heat of the body, and on the other the moisture of the surrounding parts, favour it remarkably. This putrefaction varies according to the state in which the part was at the instant of death. If much blood infiltrated it, as when inflammation destroys life, it quickly becomes putrid, blackens immediately and allows a fetid sanies to escape; this putrefaction is called moist. If there is but little blood in the part at the instant of death, its putrefaction is less sudden; it first putrifies, then blackens, and allows but little sanies to escape; this is the dry gangrene. Thus in a dead body, if one part is much loaded with blood, as the head of those that have died of apoplexy, its putrefaction is much more rapid and moist than that of the parts in which this fluid is less abundant. In the gangrene which succeeds mortification produced by cold, there is often dryness of the part, because there was but little blood in it at death. How little many physicians know of the progress of nature in the employment of antiseptics, which they apply in the living economy, as upon flesh without life. Antiseptics are applied for one of two purposes, either to prevent the death of the part, or its putrefaction. 1st. If it is with the first intention, antiseptics should be varied. By untying the artery of a limb of an animal that has been tied, you will perform an antiseptic operation. Bleeding and emollient applications which lessen the violence of inflammation in a phlegmon, are antiseptics. A tonic as wine and all stimulants which excite the vital forces in a part in which they are languid after a bruise, are antiseptics. This word is then extremely improper when it is applied to medicines designed to prevent the mortification of the parts. Antiseptics are employed to prevent a dead part in the midst of living ones becoming putrid; some effect is obtained; thus by sprinkling cinchona, muriate of soda, or any neutral salt, by moistening a limb, a portion of skin, the extremity of the nose, &c. which is dead from any cause, the putrefaction will be arrested, as in a dead body upon which the same means are employed. But what will be the result of it? a little less fetor in the surrounding parts and less danger of their receiving the influence of the emanations of the dead part; but it is always necessary that this should come off; antiseptics will never bring it to life. Hence it is evident that these means should be considered in two points of view entirely different. The first prevent mortification, and vary remarkably though they are designed to effect the same object; thus our means of curing retention of urine are very variable, oftentimes opposite, according to the cause which tends to produce this retention. The others prevent putrefaction, without restoring the parts to life; now these are uniformly the same, whatever may have been the cause of the local death.

_Action of the Air._

The air acts incessantly upon the cutaneous organ. In the ordinary state, it constantly removes from its surface the sweat that is exhaled from it. Fourcroy, who has paid particular attention to the solution of the transpired fluid by the surrounding air, appears to me to have allowed too much influence to this solution upon transpiration. In fact there are two very distinct things in this function; 1st, the action of the exhalants which throw out the fluid; 2d, the action of the air which dissolves and evaporates it. Now the first of these is wholly independent of the other. Whether the fluid is dissolved or not, more is still furnished by the exhalants. If the solution does not take place, the fluid accumulates upon the skin, which remains moist; but this moisture does not obstruct the exhalant pores and prevent new moisture from being added to it. A comparison will render this very evident. In the natural state, the serous fluids are constantly exhaled and absorbed; the absorbents perform for them the functions of the air which dissolves the sweat; now, though these vessels cease to act, as in dropsies, the exhalants continue their action; there arises only a serous collection, which, though applied to the orifices of the exhalants, does not prevent them from pouring out more serum. The bladder in vain contains urine which presses upon the opening of the ureters, these ducts do not pour less into it. Though the mucous juices become stagnant on their respective surfaces, new juices are however poured upon these surfaces. So though the skin remains moist from the want of solution of the transpiration, more transpiration is nevertheless exhaled. Solution is a physical phenomenon wholly foreign to the vital phenomenon of exhalation. We transpire in a bath as well as in the air; only the fluid which arises from it is mixed with the water, instead of being reduced to vapour.

The moisture of the skin is owing to two causes wholly foreign to each other; 1st, to the increase of the fluid furnished by the cutaneous exhalants; now the action of these exhalants may be increased from three causes. First, every thing which accelerates the motion of the heart, as running, the paroxysm of acute fevers, &c. drives to the skin, as it is commonly expressed. In the second place, every thing which tends to relax and expand the cutaneous organ by a direct action exerted upon it by the surrounding bodies, increases also the action of these exhalants, as in the great heat of summer, as in a bath and after coming out, as in a heated room, &c. In the third place, the action of the skin is in many cases, sympathetically increased. Here may be classed the sweats of phthisis of which the lungs are the source; those of fear, which depend upon a sudden affection of an epigastric organ; those of many acute diseases, &c. Now in all these cases, however active the solution by the air may be, the skin will be constantly moist, because there is thrown out upon it more fluid than the air can dissolve. Thus in catarrhs of the lungs, in which more mucous juices are thrown into the bronchia than the air can remove, it is absolutely necessary that there should be cough and expectoration to carry off the remainder.

2d. There are cases in which the moisture of the skin arises from the solution not being sufficient. This is what takes place in the moisture of the bed in which the air is not changed, in damp weather, &c. There is not then more fluid exhaled; but the ordinary fluid becomes evident, because it is not dissolved. It is under this point of view that we must consider the action of the air upon the cutaneous organ which transpires. It carries off nothing in this organ; it has no real action upon it; it takes only what its vessels throw off. Solution is merely accessory, it is always subsequent to exhalation, and has no relation with it. In the same day, in which the temperature has remained the same, the skin is often dry, moist, humid and even wet with sweat. If the air acts upon transpiration, it is by contracting or relaxing the exhalants, and not by dissolving what they throw out. If the skin formed a sac without an opening, like the serous surfaces, transpiration would go on though it was removed from the contact of the air, the same as if in contact with it. Why in fact should not that take place there, which does upon these surfaces?

If we consider the action of the air upon the skin of the dead body, we see that it produces two different effects, according to the state of the body. If the air penetrates the skin on all sides, it dries it, and it then acquires a sort of transparency, like the fibrous organs, unless a large quantity of blood had been accumulated in it at the moment of death, in which case it becomes black or of a deep brown. Thus dried, 1st, it is firm and resisting, but can be bent in various directions without breaking, as is the case with many textures thus dried, as the cartilaginous, the muscular, &c. &c. 2d. It is much less easily altered than most of the other textures in a dried state. 3d. It absorbs moisture less easily than them, though however when immersed for a long time in water, it finally resumes nearly its original colour and loses its transparency. 4th. It does not exhale a very disagreeable odour, like many of the other textures. Hence why the skins of animals, merely dried, are used in many of the arts; why some barbarous people make use of them for clothing, &c. The aponeuroses, and the mucous, serous and fibrous membranes could not be thus employed. It is to this also that must be attributed the little alteration that takes place in the exterior of mummies, which would never last for ages, if clothed with a fleshy or serous covering.

When the skin is left upon the dead body, or exposed to a moist air, it becomes putrid instead of drying. Then it takes at first a dull colour, then a green and finally a black one. It exhales a very great fetor, swells and thickens, because the gases which are disengaged there fill the cellular texture in its little spaces. A mucous covering is spread upon its external surface, which is deprived of its epidermis. Nothing similar to this covering is seen on the internal surface. Finally, when all the fluids it contains are evaporated, there remains a black residuum, very different from that which is left after combustion.

_Action of Water._

This action during life, is relative either to the substances that are deposited on the surface of the skin, or to the cutaneous texture itself.

The sweat deposits incessantly upon the epidermis many substances, the principal of which are taken away by the air, but many being slightly soluble in it, as the salts for example, remain on its surface, and adhere to it when not removed by friction. Mixed with the unctuous fluid which oozes out upon this surface, and with the different foreign particles that the air deposits there as everywhere else, these substances form upon the skin a deposit which cannot, like the transpiration, be carried off by solution. Now water removes all this deposit; hence why the use of baths is truly natural. All quadrupeds bathe themselves. All birds frequently plunge into the water; I do not speak of those for whom this fluid is as it were the element. It is a law imposed upon all species of animals whose skin throws out a considerable quantity of fluid. All the human races hitherto observed frequently plunge into brooks, rivers, or lakes, along which they take up their abode. The countries that are well watered are those which animals prefer. They avoid those where this fluid is wanting, or in which it is only sufficient for their drink. We oppose nature in every thing in society. In our own, numerous classes hardly ever use a bath; thus you must seek especially in these classes for cutaneous diseases. We have seen that the mucous juices, remaining too long upon their surfaces, irritate and stimulate them and cause there various affections. Is it astonishing that the residuum of the cutaneous exhalation which the air does not remove, should occasion various alterations upon the skin? In summer, baths are more necessary, because as many excretions are taking place by the skin, more substances are deposited there. In winter, in which every thing passes off by the urine, the cutaneous surface becomes less dirty, and has less need of being cleansed. After severe diseases, in which there has been copious cutaneous evacuations, one or two baths terminate the treatment advantageously. Let us consider water then as acting as accessory to the air upon the skin, as removing from its surface substances which the first cannot dissolve, substances, which varying remarkably like those that compose the urine, have presented the transpiratory fluids to chemists, sometimes as alkaline, sometimes as acid, oftentimes as containing salts, sometimes charged with odoriferous substances, &c. Water is the general vehicle; when it is evaporated, it leaves the substances that are not volatilized like it. It is on this account that dry frictions are also advantageous; they clean the exterior of the body.

As to the action of the bath upon the cutaneous texture, we know but little of it during life. They say in medicine that it softens, relaxes and unbends this texture; this is vague language to which no precise meaning is attached, and which is no doubt borrowed from the relaxation which the skin of dead bodies undergoes, or even tanned leather, when exposed to water. A bath acts upon the vital forces of the skin, raises or diminishes them, as I shall say; but it leaves its texture in the same state; it is only that of the epidermis which it alters, as we shall see.

Macerated in water of a moderate degree of temperature, in that of cellars for example which does not vary, the human skin softens, swells but little, becomes evidently whiter, and remains for a long time without experiencing any other alteration than that of a putrefaction infinitely less than that of the muscular, glandular, mucous textures, &c. subjected to the same experiment. This putrefaction, which removes the epidermis, appears to be much greater on the side nearest to this membrane; at the end of two or three months the skin loses but little of its consistence. It does not become pulpy as the tendons and muscles in this length of time when macerated; it does not become a fetid pulp till the end of three or four months. I have preserved some of it for eight months, which has still its primitive form, but which feels liquid under the fingers when pressed a little. In the half putrid state, the skin still preserves the faculty of crisping from the action of caloric; it moves about when placed on burning coals, or when plunged into boiling water. When once reduced to a really putrid state it loses this property.