Part 28

As we advance in age, the adhesion of the internal surface of the dermis with the subjacent cellular texture becomes much greater. It is more difficult to detach one from the other. On the external surface the wrinkles of the face are gradually formed. Smiles and tears agitate the face of the infant the most. One is the expression of the happiness, the other of the uneasiness which all its passions produce in its mind. Now the wrinkles which weeping occasions on the eyelids are marked in rather a more permanent manner, either because weeping is more frequent than smiling, or because continual winking adds to the motion which weeping produces, or because less fat is found in this place. As smiling is on the one hand more rare, and on the other much fat puffs out the cheeks of the infant, the perpendicular wrinkles formed by the muscles of the face, which in this motion separate transversely the features from within outwards, are much slower in forming. Besides, the nursing of the infant, which requires the contraction of its face from without inwards, opposes their formation. The wrinkles of the forehead are always very slow in forming, because the motions which contract the eyebrow, and those which wrinkle the forehead, are rare in the infant, who has hardly any of those dark passions which these motions serve to depict.

The growth of the dermoid system has not remarkable revolutions like that of most of the others; it goes on in an uniform manner. At the period of the growth of the hairs, it does not change, because this growth is absolutely foreign to it, these productions only passing through it. At puberty it increases in energy like all the other systems. Until then sweats had not been very copious; for, other things being equal, we may say that children sweat less in general than adults, and that the residue of their nutrition passes rather by the urine, which is probably the reason why they are so remarkably disposed to calculi. Beyond the twentieth year we begin to sweat more, and until old age, especially in summer, the fluids appear to go out in this way.

III. _State of the Dermoid System after Growth._

After growth, the skin continues for a long time to have great activity; the excess of life which animates it, renders it capable of influencing with ease the other organs if it be but a little excited. Hence the disposition to pneumonia, pleurisy, &c. from the action of cold on the skin in sweat, a state in which it is more disposed to exert an injurious influence upon the internal organs, because its forces are more excited. As to the different affections which result from this influence, they depend upon the internal organs upon which it is directed; so that the same sympathetic irradiations going from the skin, will produce sometimes an affection of the abdomen, sometimes a disease of the thorax, according to the age in which the abdominal or pectoral organs, predominating by their vitality, are more disposed to answer to the influence directed in general upon the whole economy.

The skin becomes more and more firm and resisting as we advance in age, as the fibrous substance is constantly tending to a predominance over the gelatinous. Less blood seems to be carried to it. It becomes less and less disposed to eruptions, so common in youth and infancy, &c. I will not speak of its other differences; for all that we have said of it in the preceding articles relates especially to the adult age.

I will only observe that if, during the greatest part of life, the skin be so fruitful a source of diseases, and the various alterations it experiences produce so frequent disorders in the internal organs, it is only owing to the varied causes of excitement to which it is every instant subjected. If the glands, the serous surfaces, &c. have an influence less frequently upon the other organs, it is because being deeply situated, and almost always in contact with the same excitants, they are not subject to so many revolutions in their vital forces. The secreted fluids and those exhaled in the serous and synovial systems are not, for the same reason, so much subject to those considerable increases, and those sudden suppressions which so frequently happen to the sweat.

Observe that society has also multiplied to a great extent the injurious excitements to which the skin is subjected. These excitements consist especially in the rapid passage from heat to cold, which makes the latter act very powerfully upon the cutaneous sensibility, which like that of all the other systems, answers so much the more to excitements made upon it, as they are different from those, whose action they had previously experienced. In the natural state, there is only the succession of the seasons; nature knows how to connect insensibly heat with cold, and to make the transition but rarely abrupt. But in society, the different garments, the artificial degrees of temperature of our apartments, degrees differing at first from that of the atmosphere, then varying greatly from each other, so that the same man who in winter enters thirty apartments, is often subjected to thirty different temperatures; the hard labour in which most men are engaged, and which makes them sweat copiously, every thing incessantly presents numerous causes which make the vital forces of the dermoid system vary rapidly. Thus the bronchial mucous surface is constantly in contact in cities, with a thousand excitements that are continually renewed, and with which the air is not charged in a natural state. Thus the alimentary substances, continually varying in their composition, temperature, &c. change the excitement of the gastric mucous surface, and are the source of many affections, from which most animals are exempt by the uniformity of their food.

If the skin and the mucous surfaces were always kept at the same degree of excitement by the constant uniformity of the stimuli, they would certainly be a much less fruitful source of diseases, as is clearly proved by the fœtus, which is hardly ever sick, because all the external causes which act upon its mucous and cutaneous sensibility, as the heat, the waters of the amnios and the parietes of the womb, do not vary until birth. At this period, animals brought into a new medium, find many more varieties in the stimuli which act upon them, even in a natural state and far from society; thus their diseases are naturally much more frequent after than before birth. In society, in which man has increased four, six and even ten times the number of the stimuli which affect the surfaces destined to be in contact with the external bodies, is it astonishing that the diseases should be so disproportioned to those of animals?

IV. _State of the Dermoid System in Old Age._

Towards the decline of life, the dermoid system becomes more and more firm and compact; it is softened with great difficulty by ebullition. The gelatine, which it yields, is less abundant and more hard and consistent. I think it would not be fit to make any kind of glue, even the strongest, unless mixed with that of adult animals. Its yellowish tinge becomes very deep. When it is cooled, it requires a much stronger and more durable fire to melt it; the fibrous portion of the dermis which does not melt or at least resists for a long time, is infinitely greater in proportion. It is like the bones in which the gelatinous portion is in an inverse ratio, and the earthy portion in a direct ratio to the age.

The dermoid texture becomes then like all the others, dense and stiff; it is not proper for our food, the teeth cannot tear it. Prepared with tannin, it is more resisting and less pliable, and cannot on that account serve for the same purposes as that taken from young animals. Every one knows the difference of the leather of calves and oxen, especially when the latter are old. This difference is owing first to the thickness, which being much greater in the second than the first, does not allow it to be so easily bent in different directions; and then to the nature of the texture itself. Cut in two horizontally a piece of the leather of an ox; each half will be as thin as a piece of calves skin, and yet it will be less pliable. I do not estimate here the varieties which may depend on the greater or less quantity of tannin that may be combined with it; I suppose the proportions to be all equal.

Submitted to desiccation, the human dermoid texture becomes much more stiff in old age than in the preceding ones. Maceration softens it with more difficulty. The hair of a child falls out much sooner by it than that of an old person; thus it requires longer to clean the skins of old animals than those of young ones; tanners know this very well. I would remark upon this subject, that the skins of animals, having more hairs pass through them, exhibit in comparison with that of man, an innumerable quantity of little pores on their external surface; which favours in them on this surface the action of tannin, which insinuating itself into the dermoid spaces and filling them completely with a new substance formed by the combination of tannin with gelatine, occupies entirely the texture of the spaces. The previous maceration to which the skin has been exposed, favours not only the removal of the hairs, but facilitates also to a great degree the entrance of the tannin, by separating the fibres of the spaces, by making them larger, and increasing the size of the external pores.

The more we advance in age, the less is the quantity of blood that penetrates the skin. The redness of the cheeks disappears in old people. We no longer see then the rosy complexion of the young man and even of the adult, and which arose from the vessels winding through the cellular texture of the spaces of the chorion.

The continual pressure of external objects increases then remarkably the adhesion of the subjacent cellular texture to the dermis. They are separated from each other with great difficulty by carrying the edge of a scalpel over the internal face of the chorion; a circumstance which is owing also to this, that the cellular texture having become more dense, is less easily torn; for this tearing is then necessary, considering the continuity of the sub-dermoid layer with that which enters the spaces. The exterior of the skin is uneven and rough. All the wrinkles of which we have spoken become infinitely more evident; many belong exclusively to this age.

The vital forces of the dermoid system are more weakened in old age than those of most of the others, because it is more excited during life by external bodies. Most of these bodies then make no impression upon it. The habit of feeling has blunted the animal sensibility. The touch is exercised but rarely; for, as I have observed, this sense requires to put it in action, the previous exercise of the will. We touch, because we have previously seen, heard, tasted, &c. in order to correct or confirm our other sensations; now the old man, to whom every thing around is known, to whom nothing is new, is induced to touch nothing. Compare in this respect the two extreme ages of life. The infant, to whom every thing that strikes his eyes, his ears, his nostrils, &c. is unknown, who finds in every thing that surrounds him new objects of sensation, wishes to touch and lay hold of every thing. Its little hands are in continual agitation. To touch is a pleasure to him, for every new object of sensation is agreeable. If in his last years, man was transported into the midst of objects that never before struck his senses, he would oftener exercise his touch; but none of those things excite him among which he has always lived. Hence why old age is not the age of enjoyments. In fact all our pleasures are almost relative; we have but little that is absolute; now as habit blunts all the relative pleasures, which cease because they have existed, the more the sensations are accumulated by time, the less there are of new ones left to be experienced, and the more are the sources of happiness dried up. For a contrary reason, the happiest age is infancy, because it has before it the whole field of sensations to go over. Man at every step of his career leaves behind him a cause of his enjoyments. When arrived at the end he finds only indifference, a state very suitable to his situation, since it diminishes the distance that separates life from death.

The organic sensibility of the skin is not less blunted in old age, than its animal sensibility; hence the following phenomena; 1st, contagious miasmata are absorbed with difficulty at this age; almost all pass over the cutaneous surface with impunity. 2d. The exhalation of sweat is uniformly less; it is hardly ever subject to those great increases, that are seen in the adult. 3d. The oily fluid is also furnished in much less quantity; hence the constant dryness of the exterior of the skin, the cracking of the epidermis in some cases, &c. 4th. All the diseases which suppose an increase of this organic sensibility are much more rare. Erysipelas and the different kinds of eruptions are a proof of it. When these affections take place, they have a character of remarkable slowness. 5th. The skin resists external cold much less; it loses easily the caloric of the body, which always tends to escape in order to be in equilibrium with that of the surrounding medium; thus old people are always fond of heat. 6th. I am well persuaded that the skin would resist also less, at this age, a degree of temperature greater than that of the body, and as it permits the internal caloric to be easily lost in a colder medium, it would allow the external to penetrate in a warmer one. It would be very curious to repeat, on the two extreme ages of life, the experiments of the English physicians.

EPIDERMOID SYSTEM.

If we examine attentively, it is easy to perceive the immense difference there is between the preceding system and this, which physiologists have considered as one of its dependancies. Organization, properties, composition, functions, growth, &c. every thing differs in the two. By explaining these, the line of demarcation that separates them will be perceived.

I rank in this system, 1st, the external epidermis; 2d, that which is spread upon the mucous system, or at least upon one of its parts; 3d, the nails. Though these last differ very much from the epidermis in their external appearance, yet they resemble it in so many respects, that it is difficult not to place them in the same system. In fact the nails serve as an epidermis for the skin which is subjacent to them; they are continued with that of the fingers in an evident manner, are detached and regenerated during life with the same phenomena. The composition appears to be very analogous. The kind of excrescences is the same. After death, the nails are detached by the same means as the epidermis, and then make, as it were, a part of it.

ARTICLE FIRST.

OF THE EXTERNAL EPIDERMIS.

The external epidermis is a transparent membrane, more or less thick, according to the regions in which it is examined, covering everywhere the skin, and receiving immediately the excitement of external bodies which would act too powerfully upon this.

I. _Forms, Relations with the Dermis, &c._

We see upon the epidermis the same wrinkles as upon the skin, because being exactly contiguous, both wrinkle at the same time. Different pores open on its surface after having passed through it. Some transmit the hairs; these are the most apparent; others give passage to the exhalants. We do not see these in the natural state, because their course is oblique, and they open between two small layers, which, being in contact with each other when we do not sweat, conceal their termination. But if, the skin being very dry, we suddenly sweat, as after drinking tea, then the little drops which escape from the whole cutaneous surface, not having had time to run together, but remaining separate, we see, by the places where they are, the orifice of the exhalants. Besides, if we examine against the light a considerable portion of epidermis, its transparency allows us to see many small pores separated from each other by interstices, and which pass through it in an oblique direction. It is only in the soles of the feet and the palms of the hands that we cannot make this observation, which is owing to the thickness in those parts. It is impossible to distinguish in these pores the absorbent orifices from those of the exhalants, even when mercury enters the first by friction.

The internal surface of the epidermis adheres very closely to the skin. The means of union are at first the exhalants, the absorbents and the hairs, which in passing through the first, adhere to it more or less, and thus fix it to the second, from which they arise. By separating the epidermis by maceration, which is the most proper means, we see on its internal surface many small elongations of greater or less length, and which, when examined attentively, appear to be nothing but the broken extremities of exhalants and absorbents. In fact these little elongations which are easily raised up, and which then appear like small ends of thread when they are of some size, but which exhibit only inequalities when they are left very short, have all of them an oblique course, and terminate in the pores which, we have said, pass through the epidermis to go to its surface. Their existence is sufficient, at the first inspection and without the aid of a microscope, to enable us to distinguish the internal from the external face of this membrane. The spaces that separate them are more or less large. About these spaces, the adhesions are less. It is at this place that the small epidermoid vesicles are formed with which the skin is covered when plunged into boiling water. The depressed interstices, which separate these vesicles, are the places where the exhalants are and which have prevented the epidermis from being raised up. When ebullition is long continued, they are detached also.

We cannot doubt then that all these vascular elongations serve powerfully to unite the epidermis to the chorion. How is the adhesion formed in their interstices? I know not; but it exists, though it is less evident. The cellular texture, as I have said, appears to take no part in it.

Every one knows that many causes destroy the adhesions of the epidermis, and raise it up. These causes are, 1st, every severe inflammation, whatever may be its species. We know that after erysipelas, phlegmon, biles, and cutaneous eruptions of different natures, the epidermis is always detached; there is then no fluid that raises it up. The exhalants cannot furnish it, as they are full of blood; it is dry when detached. 2d. Various cutaneous eruptions, which have not an inflammatory character, as herpes, &c. also detach the epidermis at the place where they are. It most commonly comes off then in the form of dry scales; hence no doubt the idea of some authors who have attributed to it a scaly structure, which neither observation nor experiment upon the epidermis in the natural state have proved. This pealing off in scales is owing to the same cause precisely as the formation of vesicles which take place the instant after the skin has been plunged into boiling water, viz. to the greater adhesion of the exhalant vessels which go to the epidermoid pores. Observe in fact that it is always in the space between these pores that the scales are produced, which do not exist in nature, but which arise only from the manner in which the membrane is raised up. For example, when herpetic eruptions take place on the chin, the pores through which the hairs pass are not detached; it is only the epidermis in the space between these pores; now as these are very near together, these scales are extremely small; they are almost like dust. 3d. Whenever the epidermis is raised up by cutaneous inequalities, the least friction detaches it from these inequalities. Hence why, after strong dry frictions, a rough skin becomes scaly, whilst a smooth one experiences no alteration; it is this even, which in the external appearance, contributes much to the ugliness of the one and the beauty of the other. 4th. After idiopathic fevers, and even many affections of the internal viscera, the skin which has felt the sympathetic influence of the disease, becomes the seat of an alteration which without having any external sign, is sufficient to break the union of it with the epidermis, which is everywhere raised up. 5th. We know that the action of a blister, which draws a large quantity of serum to the external surface of the chorion, breaks off the exhalants which go from it to the epidermis; so that this serum is effused under it and forms a more or less considerable sac. The water does not escape through the open pores, because their oblique course through the epidermis makes their parietes, when brought in contact with each other by the pressure of the water, form an obstacle to it. It is for the same reason, that though these pores may be very evident, as I have said, in a separate portion of epidermis when examined against the light, this portion will support mercury, without giving passage to its particles. 6th. Most of the preceding means, which produce their effect only by an alteration of the vital forces, have no effect in raising the epidermis in the dead body. Putrefaction, maceration and ebullition are those by which it is effected. All act by breaking the elongations which extend from the dermis to the epidermis, though the mechanism of this rupture is not exactly known.

II. _Organization, Composition, &c._

Authors have made many conjectures upon the structure of the epidermis, which it would be useless to relate here. I shall only speak of what accurate observation demonstrates. Its thickness is in general very uniform in all the parts. It has not appeared to me to be increased or diminished, according to the varieties of thickness of the skin on the back, the abdomen, the extremities, &c. It is only on the soles of the feet, the palms of the hands and the corresponding face of the fingers, that this thickness becomes greater. It is even so great in these places, that there is no proportion between them and the other parts of the body as it respects this membrane; it is especially towards the heel that it exhibits this character. This excessive thickness appears to be owing to different layers which are applied upon each other, and which seem to be superadded to the layer of the ordinary epidermis; but there is also a real difference, though but little known, in the organization; for example, when the epidermis has been removed from these parts by maceration, we cannot see, as in the others, those small appendices or inequalities regularly scattered over it, and which are the remains of the broken exhalants. In these places these vessels are torn smoother on the internal surface of the epidermis, on which are seen only the traces of the wrinkles of which we have spoken.

I attribute to this excessive thickness of the epidermis of the soles of the feet and the palms of the hands, the difficulty and oftentimes impossibility of making blisters act in these places, on which I have often applied them there, because I thought that the sensibility being greater, they would produce more effect in some diseases. The failure of my attempts has compelled me to renounce them.