4th. Many substances that are not medicinal can be transmitted to the blood by cutaneous absorption. Water appears to enter it in this way, in the rapid production of certain dropsies, in those cases related of travellers, who wanting fresh water on the ocean, have in part quenched their thirst by surrounding themselves with damp clothes, &c. When our garments are impregnated with the oil of turpentine, the urine soon acquires an odour that is owing to the principles transmitted to the blood by absorption. Many judicious philosophers have asserted that the weight of the body has been increased by a walk in the morning.
I have observed that after remaining in the dissecting room some time, the intestinal flatus frequently acquires an odour exactly similar to that which the bodies in putrefaction exhale. In the following way I convinced myself that it was the skin as much as the lungs that absorbed these odorous particles. I closed my nostrils, and fitted a long tunnel to my mouth, which passing out of the window allowed me to breathe the external air. The flatus from my bowels, after I had remained an hour in a small dissecting room, at the side of two very fetid bodies, acquired an odour nearly similar to theirs. I have observed also that by touching for a long time fetid substances, the flatus acquires more of this odour, than by remaining only in an air loaded with cadaverous exhalations. Then the absorbents carry at first these exhalations to the blood, which afterwards throws them out by the mucous surface of the intestines. Thus when the urine is absorbed, the saliva, the mucous juices, &c. have an urinous odour.
I could accumulate many other proofs of cutaneous absorption; but I have selected only the principal. Many others have been cited; Haller in particular, to whom I refer, has multiplied examples of it.
I would remark however that cutaneous absorptions have a character of remarkable irregularity; that under the same apparent influence, they sometimes take place and sometimes do not. It is thus that most often we absorb nothing in a bath, that we escape or take contagions, that the vaccine disease is or is not communicated, that the variolous inoculation is also often uncertain, &c. This is not astonishing. It requires a certain degree of sensibility in the skin for the absorption of this or that substance; above or below this degree, the absorbents repel this substance. Thus in the intestinal canal, if you raise by a purgative, the ordinary degree of sensibility of the lacteals, they cease immediately for a time to take up drinks, chyle, &c. and every thing passes off by the anus. Now a thousand causes act incessantly upon the skin; a thousand irritants by turns applied to it make the degree of its organic sensibility vary every instant, increase, diminish and remove it from that which is necessary for absorption. Is it astonishing then that this function should exhibit so many varieties? Many modern philosophers have produced numerous negative facts against cutaneous absorption. What do these facts prove? only the varieties of sensibility which I have noticed; but they do not destroy the mass of positive facts, generally acknowledged and which together form a body of proof which nothing can oppose. Thus we have seen the mucous surfaces variable in their vital forces on account of the variety of their excitants, vary also in their absorption. If in the serous membranes, in the cellular texture, in the work of nutrition for the organs, this function is uniform, it is because being constantly in contact with the same bodies, the surfaces where it is going on have an uniform degree of organic sensibility.
Many facts, in relation especially to contagions seem to prove that a state of weakness is favourable to cutaneous absorption. 1st. Children and women absorb more easily than strong and vigorous men. 2d. Many physicians have observed that in the night in which the cutaneous organ is in a state of remission in this respect, as it is not stimulated by external objects, contagious diseases are more easily taken. 3d. I have remarked that most of the pupils who have fallen sick during my dissections, had carried to their chambers portions of subjects, the emanations from which had been able to affect them during sleep. 4th. We know that practitioners recommend, that persons should not expose themselves to contagious miasmata during hunger, as the forces are then languid on account of the emptiness of the stomach.
Exhalants.
The external capillary system which surrounds the chorion and embraces the papillæ, appears to be the origin of these vessels, as it is the termination of the arteries of the dermoid spaces. The exhalants take up their fluid there, which they throw out upon the epidermis. We have no anatomical knowledge as to their form, their length, their course and their direction; but their existence is incontestibly proved, 1st, by injections, which are sometimes poured out upon the whole cutaneous surface; 2d, by the sanguineous exhalation which takes place in some diseases in which there is a real bloody sweat; 3d, by the natural sweat and by transpiration, which can evidently have no other agents, though some authors have admitted that there were certain pretended glands for the secretion of these fluids.
An infinite number of calculations has been made to ascertain the quantity of fluid which the cutaneous exhalants usually pour out. We are dismayed when we read the result of the labours of many philosophers upon this point, when we go over the calculations, enormously multiplied, of Dodard, Sanctorius, Keil, Robinson, Roye, &c. To what do all these calculations, for which the life of a single man would perhaps be insufficient, tend? To prove to us that when we start from a false principle, the whole chain of consequences drawn from it is false, though these consequences may be rigorously deduced from each other. In fact, most philosophers have considered the skin as a kind of fountain with numerous capillary tubes, always throwing out in the same time the same quantity of fluids, and being able consequently to be subjected, like inert capillaries which pour out fluids, to proportions and calculations of quantity. But the results of these calculations have soon proved how mistaken their authors were. Read these results, and you will see that none of them agree, that frequently very great differences characterize them. Is this astonishing? A thousand causes make the transpiration vary at every instant. Temperament, exercise, rest, digestion, sleep, watchfulness, the passions, &c. increase or diminish the action of the cutaneous exhalants. I do not speak of the difference from climate, seasons, &c. which is still more decided.
An attempt has even been made recently to ascertain, what belongs to the urine, to the transpiration, to the pulmonary perspiration and to the excrements, to calculate the relation which exists between the quantities thrown out in these four ways; useless researches! We might obtain from them results for one man, which would not be applicable to others. Thus see if we have ever been able to make a single useful application to physiology or pathology of all these immense labours on transpiration. What would you say of a man who, during the days of the equinox, in which the state of the atmosphere was every minute changing, should try to establish proportions between the quantities of rain which fell in every quarter of an hour, or of one who endeavoured to fix relations between the quantities of fluids which are evaporated in given times, from the surface of a vessel under which the intensity of the heat which warmed the water varied every instant. The comparison is just. We might be able to say in general, at the end of a given time, nearly how many pounds of substances went from the body; and yet this varies in every individual. But to attempt to say in a general manner what, in this common quantity, the urine and transpiration separately furnish, is to prove that we do not understand the nature of the vital forces.
We have already observed, that all our knowledge upon the varieties of transpiration, is reduced to some general data; that, for example, in cold seasons and climates, it is by the internal emunctories that the residue of nutrition and digestion principally passes off, whilst in warm climates and seasons, it is the cutaneous organ that principally throws it out.