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.