[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 choledochus[A].