When all the internal functions are in activity, the mucous surfaces experience no more sudden changes, analogous to that of which I have spoken. They grow like the other viscera in a slow and insensible manner; they preserve for a long time their original softness, which is remarkable, especially in the nose, the stomach, &c. and which during lactation, is not adapted in the infant, to the solid substances with which the adult is nourished. Is this softness the cause of the mucous affections which are in general so common at that age? We know that then the mucous juices abound; the pituitary membrane is more moist; the stomach and intestines are frequently affected with a species of catarrh which is the cause of the looseness that we have so often to combat in infancy. The membrane of the bronchia is also frequently diseased. The two extreme ages of life resemble each other by the abundance of the mucous juices secreted upon their respective mucous surfaces.
In youth the mucous system is in very powerful action. The active hemorrhages of this system are very frequent at this age; those of the nose, the bronchia and even the stomach often take place; those of the portions of this system, subjacent to the diaphragm, are then less common. Observe that in man, hemorrhages of the gastro-pulmonary surface are infinitely more frequent than those of the genito-urinary surface, which on the contrary, are much more numerous in woman in whom one of them is natural to a part of this surface, viz. menstruation.
At the period of puberty, the development of the genital parts in both sexes, gives much activity to a part of the genito-urinary surface; then menstruation begins upon that of the womb; then the sensibility of the urethra is raised in order to feel acutely the passage of the semen. Observe that this increase of energy is not attended with a weakness of the other parts, as happens in many cases; on the contrary, all the systems, all the apparatus seem to borrow, from the force which the genital parts acquire, an increase of action.
II. State of the Mucous System in the subsequent Ages.
In the years which succeed youth, the mucous system continues to grow, thicken and become firmer. Its vital energy seems still to predominate for some time, in the superior surfaces, as in the pituitary, the membrane of the bronchia, &c.; thus the affections of these parts are more frequent until the thirtieth year. But as we advance in age, the abdominal mucous surfaces appear to predominate over the others, as in general all the organs of this region do.
Besides, a thousand causes in the course of life, make the state of the mucous system vary. We do not find it in two subjects, with the same shade of colour, with the same density, with the same external appearance. By taking any surface upon many subjects, that of the stomach, for example, we easily see these differences, with which we must be struck if we have opened dead bodies but ever so little.
The redness of the mucous texture is very bright until the thirtieth year; after that, it begins to alter. This texture becomes more and more pale in old age; the blood enters it but in small quantity; it acquires more consistence and density. The fingers carried over it no longer perceive that softness, that velvet so remarkable in the first age. Its forces, which grow languid, render difficult, in the excretories, the exit of the fluids which pass through these tubes to be thrown out. Yet the mucous glands still secrete their fluids in very great abundance. Often even these fluids increase in proportion which constitutes the catarrhal affections, so common in old age. But these affections then have the same character as the functions of the whole system; secretion takes place slowly; the disease is always chronic; most often it terminates only with life.
The mucous absorption is, at this age, slow and difficult, like all the others; the various contagions are taken much less easily, either by the respiratory surfaces, or by the contact of contagious miasmata upon the neighbouring surfaces of the skin. The chyle slowly absorbed, makes the digestive periods longer.