[SECTION VII.]
OF THE VITAL POWERS OF THE MUCOUS MEMBRANES.
57. The sensibility of mucous membranes is one of the principal characteristics that distinguishes them from other analogous organs. This power, which belongs to organic bodies, is variable in every part, prompt to develop itself in some parts, under the influence of the least excitement, roused with difficulty in others, present in every part, liable to proceed by means of inflammation from the most obscure state to the last degree of intensity—this power is here remarkable for features very analogous to those which it presents in the cutaneous surface (to which, as we have stated, the mucous surface has great traits of resemblance) as respects its structure. It is to this analogy of sensibility that we must refer a crowd of phenomena, which are alternately exhibited in an inverse order upon both surfaces. I shall now point out some of these phenomena in succession.
58. (1) When the temperature of the surrounding air deadens the sensibility of the cutaneous organ, by contracting its tissue, the sensibility of the mucous surface receives a remarkable increase of energy. Observe why in winter, and in cold climates, where the functions of the skin are singularly limited, all those of the mucous membranes are in proportion augmented; thence arises a more evident pulmonary exhalation, the internal secretions are more abundant, digestion is more active and more ready to operate, consequently the appetite is the more easily excited. (2) When, on the contrary, the heat of the climate, or of the season, &c. relaxes and opens the cutaneous surface, we should say, that the mucous surface is in proportion constricted: during summer, in the south, &c. there is a diminution in the internal secretions, the urine for instance; a tardiness in the digestive phenomena by a default in the actions of the stomach and intestines, and the appetite is slow in returning. (3) The sudden suppression of the functions of the cutaneous organ often determines a morbid increase of action in those of the mucous membrane. Cold air, which checks the perspiration, frequently produces colds and catarrhs, affections which are marked by the sensibility and increased action of the mucous glands. (4) In various affections of the mucous membranes, baths, which relax and determine to the skin, produce beneficial effects.
59. The foregoing considerations evidently establish the influence, which the vital powers of the skin have over those of the mucous membranes. Others, not less important, demonstrate the reciprocal dependence in which the skin is found with the same membranes, as respects their vital powers. (1) During digestion, when the mucous fluids are poured out in abundance into the stomach and intestines, when, consequently, the mucous membranes of the alimentary canal are in high action, the fluid of insensible perspiration is evidently diminished, according to the observation of Santorius: it is very small in quantity three hours after a meal, so that the action of the cutaneous organ is visibly less energetic. (2) During sleep, when all the internal functions become more marked and are in full action, at which time the sensibility of the mucous membranes is consequently highly excited, the skin appears to be seized by a manifest debility—a debility, which is evinced by the cold which it experiences when the animal reposes at night uncovered, and by its want of susceptibility of various impressions.
60. The sensibility of the mucous membranes, like that of the cutaneous organ, is essentially submissive to the immense influence of habit, which, tending incessantly to blunt the acuteness of the sensations of which they are the seat, reduces the pain and the pleasure that we receive through them equally to indifference, which is, as some say, the middle state.
61. I say, in the first place, that habit reduces the painful sensations, which take place on mucous membranes, to indifference. The presence of the catheter, which is passed up the urethra for the first time, is cruel the first day, painful the second, inconvenient the third, scarcely felt the fourth; pessaries introduced into the vagina, bougies into the rectum, tents in the nasal fossæ, the canula in the nasal canal, produce, in different degrees, the same phenomena. It is upon this remark that is founded the possibility of introducing instruments into the trachea to aid respiration, and into the œsophagus to afford artificial deglutition. This law of habit may even transform a painful into a pleasant impression; of this fact the use of snuff, tobacco, and various kinds of food, furnish us with remarkable examples.