39. Let us conclude, from these numerous considerations, that one of the principal means that nature employs to augment the action of the glands, and to excite that of their excretory ducts, is irritation upon the extremities of these ducts. We must refer to that cause the abundant secretion and excretion of mucous fluids in the cases above stated. It is also to this susceptibility of the mucous glands, to be excited by irritation at the extremities of their excretory ducts, that we must attribute the artificial catarrhs which are occasioned by the respiration of chlorine gas; the flow of mucus which attends a polypus, any tumour in the vagina, stone in the bladder, &c. The frequent occurrence of leuchorrhea in women who use coition immoderately, the abundant flow of mucus from the noses of those persons who take snuff, in all these cases there is evidently an irritation of the mouths of the mucous ducts.
40. The mucous membranes, by the continual secretion of which they are the seat, perform a principal part in the animal economy. They are to be regarded as one of the grand emunctories, by which the residue of the nutriment constantly escapes from the body; and consequently as one of the principal agents of that habitual decomposition which carries away from living bodies the particles which for some time formed part of the solids, but have at length become heterogeneous to them.
41. Remark the fact, that none of the mucous fluids enter into the circulation, but are thrown out externally; that of the bladder, ureters, and urethra, with the urine; that of the vesiculæ seminales and of the vassa defferentia with the semen; that of the nostrils by the action of blowing the nose; that of the mouth partly by evaporation, and partly by the anus with the excrements; that of the bronchi by the pulmonary exhalation, which is effected principally by the solution of this mucous fluid in the air of respiration; those of the œsophagus, of the stomach, of the intestines, of the gall bladder, &c., with the excrements of which they frequently form, in the ordinary state, a part nearly equal to the residue of the aliment; and they even compose almost the whole of the matter voided in certain dysenteries and fevers, where the quantity is evidently disproportionate to the food that has been taken. Let us observe on this subject, that in the analysis of the fluids, in contact with the membranes of which we speak, as the urine, bile, gastric juice, &c., there are always some errors, because it is very difficult, impossible even, to separate them from the mucous fluids.
42. If we call to mind what has been said above, upon the extent of the two general mucous surfaces, that they are equal and even superior to the extent of the cutaneous organ; if we afterwards contemplate these two grand surfaces, constantly throwing off the mucous fluids, we shall see of what importance this evacuation must be in the economy, and of what derangements its lesion may become the source. It is doubtless to this law of nature, which ordains that every mucous fluid shall be rejected externally, that in the fœtus we must attribute the presence of the unctuous fluid, of which the gall bladder is full, and of the meconium choking up the intestines, &c., kinds of fluids which appear to be only a collection of mucous juices, which, as they cannot be evacuated, remain, until birth, upon the organs where they have been secreted.
43. It is not the mucous fluids only that are rejected externally; almost all the fluids, separated from the mass of blood by the means of secretion, have the same destiny: this is evident in the most considerable part of the bile. It is very probable, also, that the saliva, the pancreatic juice, and the tears, are discharged with the fæces, and that it is their want of colour alone that prevents them from being distinguished like the bile. I do not know even if, in reflecting on a crowd of phenomena, one would not be tempted to establish, as a general principle, that no fluid, separated by secretion, returns into the circulation; that this destination belongs only to fluids separated by exhalation, as those of the serous cavities, of the articulations, of the medullary organ, &c.; that all the fluids are thus excremental or recremental, and that there is no recremental excrement, as the common division points out[B].
44. What is certain, at least, is, (1) that I have never been able to effect the absorption of bile or saliva by the lymphatics. When I have injected them into the cellular tissue of an animal they have always produced inflammation and suppuration. (2) We know that the urine, when infiltrated, does not become absorbed, and that it strikes with death every part that it touches; whilst the infiltrations of lymph, or of blood, are readily absorbed. (3) There is an essential difference between the blood and the secreted fluids as concerns their decomposition, whilst exhaled fluids and serum, &c., are in that respect very similar.