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SECTION IV.
OF THE GLANDS OF MUCOUS MEMBRANES

34. Besides the three strata, which we have just mentioned, the mucous membranes present in their structure a great number of glands and blood vessels. The mucous glands exist in all membranes which bear that appellation: they are situate under their chorion, and even in its substance: they continually discharge, through imperceptible orifices, a mucilaginous fluid, which lubricates their free surface, and defends it from the impression of the bodies with which it is in contact, at the same time that it facilitates the passage of those substances.

35. These glands, which are very apparent in the bronchi, palate, œsophagus, and intestines, where they take the name of the anatomists who have particularly described them, are less obvious in the bladder, the gall bladder, uterus, vesiculæ seminales, &c.; but the mucus which moistens the membranes unequivocally demonstrates their existence. In fact, since this fluid is nearly of the same nature on all the mucous surfaces, and, in those where the glands are apparent, is evidently furnished by them, it must be secreted in the same manner in those where they are less evident. The identity of secreted fluids, certainly, supposes the identity of the secreting organs. It should seem, that in situations where these glands escape our observation, nature makes up for their tenuity by increasing their number. In the lower animals, particularly in the intestines, they form by their number a kind of new layer, in addition to those we have described. The same may be observed in the palate, velum, &c. in man.

36. There is therefore this great difference between mucous and serous membranes; that the fluid which lubricates the former is furnished by secretion, whilst that which moistens the latter is produced by exhalation. We know but little of the composition of mucous fluids, because in the natural state it is difficult to collect them, and in the morbid state, where their quantity increases, as for instance in catarrhs, their composition probably undergoes some alteration: but their functions in the animal economy are well ascertained.

37. The first of these functions is to defend the mucous membranes from the impressions of the bodies with which they are in contact, and which, as we have observed, are all heterogeneous to the animal. Here, without doubt, we see the reason why the mucous fluids are more abundant in the cavities where these bodies remain for some time, as in the bladder, at the extremity of the rectum, &c., than in those organs through which they merely pass, as in the ureters, and in general in all the excretory ducts. Observe again, why, when the impression of these bodies might be hurtful, these fluids are poured out upon their surfaces in a much greater quantity. The sound which is introduced into the urethra, and is allowed to remain there; the instrument that is left in the vagina to secure a polypus; that which, with a similar intention, remains some time in the nasal fossæ; the canula, fixed in the lacrymal sac, to remove the obstruction; and the tube that is introduced into the œsophagus, when deglutition is interrupted, always determine a more plentiful secretion upon the corresponding mucous surface. This is one of the principal causes why it is so difficult to retain elastic tubes in the trachea; the abundance of mucous fluid, which is then separated, chokes up the apertures of the instrument, and renders its frequent removal necessary, and may even threaten the patient with suffocation, as Desault has himself observed, although he has nevertheless many times succeeded with that operation.

38. It therefore appears, that every acute excitement of mucous surfaces determines, in the corresponding glands, a remarkable augmentation of action. But how can this excitement, which does not take place immediately upon the glands, have so great an influence over them? For, as we have said, these glands are always subjacent to the membrane, and are consequently separated by it from the irritating bodies. It appears that the above fact belongs to a general modification of the glandular sensibility, which is susceptible of being put into action by every irritation upon the extremities of the excretory ducts, which will be proved by the following considerations: (1) The presence of food in the mouth produces a more abundant flow of saliva. (2) The catheter fixed in the bladder, and irritating the ureters, or their vicinity, increases the flow of urine. (3) The introduction of a bougie, but half way up the urethra, will often be sufficient to occasion the bladder to contract with a power equal to force the urine through the passage, and so to overcome an obstruction in the canal. (4) The irritation of the glans, and of the extremity of the urethra, sub coitu, determines the contraction of the vesiculæ seminales, and augments the secretory action of the testes. (5) The action of an irritating fluid on the tunica conjunctiva occasions an abundant flow of tears. (6) In making experiments upon the state of the abdominal viscera during digestion, and under the influence of hunger, I have observed, that whilst the food is only in the stomach there is very little flow of bile; but it increases when the aliment passes into the duodenum, so that then there is a considerable quantity in the intestines. During hunger the gall bladder is distended, and but little bile flows into the intestines. At the end of digestion, and even when that process is half finished, the gall bladder contains but half of its full quantity; yet it might be expected to empty itself more easily during abstinence, for then the bile it contains is of a deep green colour, very bitter, very acrid, and likely to irritate the organ which encloses it. On the contrary, during, or immediately after digestion, it is more clear, mild, and less irritating; there must, therefore, be, during digestion, another stimulus: now this stimulus is the aliment passing over the mouth of the ductus communis choledochus¹.

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 out².

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.