Treatise on the Anatomy and Physiology of the Mucous Membranes

55. With regard to the particular structure of that portion of mucous membrane which lines the sinuses I remark, that it is absolutely the same as of that which is spread over the surface of the internal ear, with the exception of a still more delicate tissue. All anatomists call this membrane the periosteum of the bony covering of the internal ear. The following considerations prove that it is not a fibrous membrane, analogous to that which covers the bones, but a mucous layer, like that of the sinuses. (1) It is evidently seen to be a continuation of the pituitary membrane by the medium of the Eustachian tube. (2) It is found to be habitually moist with a mucous fluid, which is discharged through that tube, a property foreign to fibrous membranes, both of whose surfaces are always attached to some parts of the animal structure. (3) No fibre can be distinguished in it. (4) Its spongy appearance, though whitish, its softness, the readiness with which it gives way to the least agent directed against it, with a view to tear it, form a character not to be found in any part of the periosteum.

56. I pass over the other variations of structure in mucous membranes in their different regions; in all they have real differences. I observe only, (1) That these variations distinguish them from serous membranes, whose aspect is everywhere the same, as may be seen by comparing the pericardium with the peritoneum, &c. (2) The sensibility of mucous membranes varies in a very peculiar manner in their different portions: thus an emetic irritates the stomach, but not the conjunctiva; the pituitary membrane perceives only odours; the mucous surface of the tongue flavours, &c. On the contrary, the contact of all kinds of bodies with the naked serous membranes produces phenomena exactly analogous.

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.

62. In the second place I observe, that habit produces indifference to those sensations on the mucous membranes which were at first agreeable. The perfumer placed in a fragrant atmosphere, and the cook, whose palate is constantly affected by delicious flavours, do not experience, in their professions, the exquisite pleasures that they prepare for others. Habit may even change pleasant sensations to painful ones, as in the preceding paragraph we saw it changed painful to pleasing sensations. I observe, further, that this remarkable influence of habit is exercised only over sensations produced by simple contact, and not over those produced by real lesion of the mucous membranes: thus it does not ameliorate the pain produced by stone in the bladder, nor that which attends polypus in the uterus.

63. It is to this power of habit over the vital energies of the mucous membranes that we must, in part, refer the gradual diminution of their functions which accompanies advancing age. All is susceptibility in the infant: in old age all is dull. In the one the very active sensibility of the alimentary, biliary, urinary, and salivary mucous surfaces, is that which principally produces that rapidity with which the digestive and secretory phenomena succeed each other. In the other this sensibility, weakened by the habit of contact, does not so closely connect the same phenomena.

64. Does not the following remarkable modification of the sensibility of the mucous surfaces depend upon the same cause, viz. that at their origin, as on the pituitary membrane, the glans, the anus, &c., they give us the sensations of bodies with which they are in contact, and that they do not produce this sensation in the deeply seated organs which they line, as the intestines, &c.? In the interior of these organs this contact is always uniform; the bladder is in contact with the urine only, the gall bladder with the bile, the stomach with the aliments masticated and reduced to an homogeneous, pulpy paste, whatever may be their diversity. This uniformity of sensation prevents perception, because, in order to perceive, we must compare, and here two terms of comparison are wanting. Thus the fœtus has no sensation of the liquor amnii: the air is also very irritating at first to the new-born infant, but at length it is not felt. On the contrary, at the origins of mucous membranes exciting agents vary every instant: the mind can, therefore, perceive their presence, because it is able to establish relations between their various modes of action. What I say is so true, that if in the interior of the organs the mucous membranes be in contact with a foreign body differing from that which is habitual to them, they transmit the sensation of it to the mind; instruments introduced into bladder or stomach are examples of it. Fresh air, which in very hot weather is suddenly introduced into the trachea, causes an agreeable sensation over the surface of the bronchi; but from habit we soon become insensible to it, and the perception ceases.

65. It is very difficult to point out with precision the character of the tonic powers of mucous membranes, because, being almost in every part united to a muscular layer, we can hardly distinguish what belongs to the tonicity of the one from what depends upon the irritability of the other; or otherwise, if the mucous membranes be isolated, as in the nostrils, yet their attachment renders the phenomena of their tonic powers very obscure. Nevertheless, the action of the excretory ducts on their respective fluids, that of the gall bladder, and of the vesiculæ seminales, which are destitute of muscular attachments, and the spasmodic contraction of the urethra, which sometimes takes place when the sound is introduced, leave no doubt of the energy of this tonic power, doubtless similar in its various modifications to that which is observed in the cutaneous organ.

SECTION VIII.

OF THE SYMPATHIES OF MUCOUS MEMBRANES

66. I distribute the sympathies of mucous membranes, like those of most of the other organs, into three general classes. In the first class are ranked the sympathies in which irritation, on one part of the mucous surface, produces a sensation in a distant part. A stone in the bladder occasions pain at the end of glans; worms in the intestines excite an itching at the nose. Whytt has seen a painful affection induced over the whole side of the head by a foreign body in the ear; an ulcer in the bladder produces a pain in the superior parts of the thighs every time that the patient passes his urine.

67. I refer to the second class those sympathies in which the irritation of one point on mucous surfaces produces irritability in a different structure; thus, too lively an impression on the pituitary membrane occasions sneezing; the irritation of the bronchi coughing; biliary concretions produce spasmodic vomiting; stones in the bladder occasion retraction of the testicle towards the ring. In all these cases there is contraction of the muscles produced by the irritation of the mucous surface, distant from the place in which that contraction occurs.

68. The last class of the sympathies of mucous membranes embraces those in which the irritation of any part of their extension determines elsewhere the exercise of their tonicity. Here we must refer to what we have said upon glandular action being augmented by the irritation of the extremities of the excretory ducts. Thus it is evident, that the increase of the tonic power of the parotid for the secretion of the saliva, and of its excretory duct in order to transmit it, when the extremity of this duct is irritated by food, sialogogue medicines, &c., – it is evident, I say, that this augmentation is a phenomenon purely sympathetic. We may designate each of these three classes by the name of the vital power which they bring into action, calling the first sympathy of sensibility; the second, sympathy of irritability; and the third, sympathy of tonicity.

69. This manner of classing the sympathies is entirely borrowed from the state of the vital powers, of which they are but irregular modifications, and only aberrations, still unknown in their nature. Nevertheless it is subject to very great inconveniences: yet it appears to me to be preferable to that of Whytt, who simply follows the order of the regions; and even to that of Barthy, who, more methodical, examines them successively in the organs connected by systems, in those which are insulated, and in those situated in symmetrical halves of the body.

SECTION IX.

OF THE FUNCTIONS OF MUCOUS MEMBRANES

70. I have already examined many of the functions of mucous membranes. I have considered them (1) As one of the grand emunctories of the animal economy. (2) As performing the same functions with respect to heterogeneous bodies, which may be within our organs, as the skin does with regard to the bodies with which it may be in contact. (3) As facilitating the passage of foreign bodies by means of the mucous fluid by which they are lubricated. It remains for me to examine three questions much agitated at this time. (1) If the mucous membranes have any influence over the redness of the blood. (2) If they exhale. (3) If the absorbents arise from them; and if absorption consequently takes place there.

71. The remarkable redness of these membranes, the analogy of respiration, during which the blood becomes changed in colour through the mucous surface of the bronchi, the well-known experiment of a bladder filled with blood and placed in oxygen gas, by which this fluid becomes also changed in colour, – have led to the belief, that the blood, being separated from the atmospheric air merely by a very fine pellicle on certain mucous surfaces, as the pituitary membrane, the palatine, the glans, &c., would there also take a brighter red colour, either by parting with a portion of carbonic acid gas, or by combining with the oxygen of the atmosphere, and that these membranes thus fulfilled functions accessory to those of the lungs. The experiments of Jurine upon the cutaneous organ, experiments adopted by many celebrated physicians, appear also to favour the reality of that conjecture.

72. Observe the experiment that I have tried, in order to ascertain the validity of that fact. Through a wound in the abdomen I drew out a portion of intestine, which I tied at one point. I then returned it, keeping back a part, which I punctured, and introduced into it sufficient atmospheric air to distend all that portion of the bowel between the ligature and the orifice. I then confined the air by another ligature, and reduced the whole. At the end of an hour the animal was opened. I compared the blood of the mesenteric veins, which arise from that portion of intestine distended by air, with the blood of the other mesenteric veins arising from the remainder of the canal: no difference of colour could be observed: the internal surface of the inflated intestine did not exhibit a brighter red. I expected to obtain a more marked effect by repeating the same experiment on another animal with oxygen gas, but I did not perceive any variation in the colour of the blood. As on the mucous membranes, which are ordinarily in contact with the air, this fluid is constantly renewed, and is agitated by a perpetual movement, I tried to produce the same effect in the intestines; for which purpose I made two openings into the abdomen, through each of which I drew a portion of the intestinal tube. I opened these two portions, adapting to one the tube of a bladder filled with oxygen gas, and to the other that of an empty bladder. I then pressed the full bladder so as to make the oxygen gas pass into the empty one through the intermediate portion of intestine which was in the abdomen, so that the warmth there might encourage the circulation. The oxygen gas was in this manner sent many times from one bladder to the other, making a current through the intestine, which from its contraction was more difficult than it at first appeared to be. The abdomen was then opened, but no difference was found between the venous blood returning from that portion of the intestine, and that which flowed from the other parts of the canal. The superficial situation of the mesenteric veins, which are covered by only a fine transparent lamina of peritoneum, and their volume when the animal is not fat, render these comparisons very easy to be made.

73. I think, that from what occurs in the intestines we cannot infer what takes place in the pituitary and palatine membranes, &c.; because, although analogous, their organization may be different. In these parts we cannot examine the venous blood returning from them, as in the intestines: but, (1) If we consider, that in animals, which have for some time respired oxygen gas, the mucous membrane of the fauces does not exhibit any increase of redness; (2) If we bear in mind, that the lividity of different parts of this membrane, in those asphyxias which are produced by carbonic acid gas, is not occasioned by the immediate contact of this gas with the membranes, but by the reflux towards the surface, of the venous blood which cannot pass through the heart, as occurs in submersion, as demonstrated by Godwin, and as takes place in all those cases in which the blood, previous to death, has found difficulty in passing through the lungs; (3) If we remark lastly, that in these circumstances the contact of the air, after death, does not alter the lividity that the venous blood gives to the mucous membranes, although the skin is then more permeable to every kind of æriform fluid; – we shall see that we must at least suspend our judgment, respecting the colouring of the blood through mucous membranes, until farther observations shall have decided the question.

74. Observe another experiment, which may throw more light still upon the subject. I have distended the peritoneal cavity of different Guinea pigs with carbonic acid gas, with hydrogen gas, with oxygen gas, and with atmospheric air, to see if I could obtain, through a serous membrane, what I had not been able to effect through a mucous surface. In these experiments I have found no difference in the colour of the blood of the abdominal system: it was the same as in fresh animals of the same kind, that I always used to compare with those on which the experiments were made.

75. I believe, nevertheless, that I have observed many times, both in frogs and in animals with warm and red blood, such as cats and Guinea pigs, that the infiltration of oxygen gas into the cellular tissue gives, after a certain time, a brighter colour to the blood than this fluid presents in the artificial emphysemas which may be produced by carbonic acid gas, hydrogen gas, or by atmospheric air, in which circumstances the blood differs very little in colour from its natural shade. But in other cases oxygen gas has had no influence over the colour of the blood; so that, notwithstanding the many experiments that have been made on this point, I cannot state any general result. It appears, that the tonic powers of the cellular tissue, and of the coats of the vessels which ramify in it, receive a very varied influence from the contact of the gases, and that, according to the nature of that influence, the fibres contracting and becoming more or less firm render these parts more or less permeable, both to the æriform fluids, which have a tendency to escape from the blood to unite with that of the emphysema, and to this last fluid, if it tends to combine with the blood. This will doubtless explain the variations that I have observed.

76. Do the mucous surfaces exhale? The analogy of the skin would seem to lead to the belief of it; for it appears well proved, that the perspiration is not a transudation by the inorganic pores of the cutaneous surface, but a true transmission by vessels of a particular nature, and continuous with the arterial system.

77. It appears, at first, that the pulmonary perspiration which takes place on the surface of the bronchi, which has such connection with that of the skin, which increases or diminishes according to the decrease or augmentation of the other, and of which the composition is apparently of the same nature – it appears, I say, that the pulmonary perspiration is produced, at least in part, by the system of exhalent vessels; and that if the combination of the oxygen of the air concurs with the hydrogen of the blood to produce it, during the act of respiration, it is but in a very small quantity, and for that portion only which is purely aqueous. It is necessary to observe further on this subject, that the dissolution of the mucous fluid, which lubricates the bronchi, in the air that is constantly inspired and expired, furnishes a considerable portion of that vapour which rises from the lungs, and which is insensible in summer, but very apparent in winter.

78. The intestinal juice, that Haller has particularly considered, but which appears to be less in quantity than he had estimated, the gastric juice, and that of the œsophagus, are very probably disposed of by way of exhalation on their respective mucous surfaces; but in general it is very difficult to distinguish with precision, in these organs, what belongs to the exhalent system from what is furnished by the system of mucous glands, which, as we have said, are everywhere subjacent to them. Thus we constantly see the mucous fluids of the œsophagus, stomach, and intestines, mix themselves with the other fluids of these parts.

79. That mucous membranes absorb is evidently proved by the absorption of the chyle upon the intestinal surfaces, of venereal virus upon the glans and urethra, of variolous poison which is sometimes rubbed upon the gums, of the serous portions of the bile, of the urine, and of the semen, when they remain in their respective reservoirs. When, from paralysis of the fleshy fibres which terminate the rectum, the fæces accumulate at the extremity of that intestine (a very common case in aged persons, and of which Desault has cited many instances), these accumulations frequently become hard, probably from the absorption of their juices, which are obstructed there. We have many cases in which the urine has been almost entirely absorbed by the mucous surface of the bladder, when there has been absolute obstruction in the urethra. Whatever may be the mode of this absorption, it appears that it is not performed in a constant, uninterrupted manner, like that of the serous membranes, in which the exhalent and absorbent systems are in a continual alternate action; but that it occurs only under certain circumstances, of which perhaps the greatest part are not in the natural order of the functions. Finally, we have yet fewer data respecting the mode of mucous absorption than on that of cutaneous absorption: we confess it is very little understood, and many even question its existence.

SECTION X.

REMARKS ON THE AFFECTIONS OF MUCOUS MEMBRANES

80. It is not my design to examine the affections of mucous membranes; I shall notice only some phenomena, which in these affections I believe deserve a particular attention, and the explanation of which I propose to physiological physicians.

81. Why do mucous membranes seldom contract adhesions from inflammation, since that occurs so frequently in serous surfaces under the same circumstances? Why does not the internal surface of the inflamed stomach, intestines, or bladder, adhere in its various portions like the pleura, tunica vaginalis, testis, &c.

82. Why, in inflammations of mucous membranes, is there an abundant flow of that fluid which habitually moistens them, and which constitutes the different kinds of catarrhs, whilst the source of the fluid that exhales from serous membranes is generally dried up in analogous cases?

83. Why do polypi, a kind of affection peculiar to mucous membranes, seldom arise but at the origins of these membranes in the vicinity of the skin, as in the nose, pharynx, vagina, &c., and not in their more internal portions, as in the stomach, intestines, &c.? Does this arise from the peculiarity of the texture that I have shown mucous membranes to have in the vicinity of those places where they arise from the skin, or must we attribute this fact to the more numerous causes of irritation which act upon the origins of these cavities?