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SECTION V.
OF THE VASCULAR SYSTEM OF MUCOUS MEMBRANES

45. The mucous membranes receive a great number of vessels: the remarkable redness which distinguishes them would be sufficient to prove it to us, if it could not be demonstrated by injections. This redness is not everywhere uniform; it is less in the bladder, large intestines, and frontal sinuses; very marked in the stomach, small intestines, and vagina, &c. It is produced by a web of very numerous vessels, whose supplying branches, after having passed through the chorion, finish on its surface by an infinite division, embracing the corps papillaire, and is covered only by the epidermis.

46. It is the superficial position of these vessels that frequently exposes them to hæmorrhages, as we remark principally in the nose, and as is seen in hæmoptysis, hæmatemæsis, hæmaturia, in certain dysenteries, where the blood escapes from the parieties of the intestines, in uterine hæmorrhages, &c.; so that those spontaneous hæmorrhages, which are independent of any external violence applied to the open vessels, appear to be special affections of the mucous membranes; they are seldom observed but in these organs, and they form at least one of the grand characteristics which distinguishes them from all the other membranes.

47. It is also the superficial situation of the vascular system of mucous membranes that renders their visible portions, as on the lips, the glans, &c.; serviceable in showing us the state of the circulation. Thus, in various kinds of asphyxia, in submersion, strangulation, &c., these parts present a remarkable lividity; the effect of the difficulty that the venous blood finds in passing through the lungs, and of its reflux towards the surfaces where the venous system arises from that of the arteries.

48. I have already observed in the fœtus, and newly born infant, that the vascular system is as apparent in the cutaneous organ as in the mucous membranes; that the redness is there the same; it is even in that part more marked in the earlier periods of conception; but soon after birth all the redness of the skin seems to concentrate itself upon the mucous membranes, which before, being inactive, had no need of so considerable a circulation, but which, becoming all at once the principal seat of the phenomena of digestion, of the excretion of the bile, of the urine, of the saliva, &c., demand a larger quantity of blood. The long continued exposure of mucous membranes to the air frequently occasions them to lose their characteristic redness, and they then assume the colour of the skin (as M. Sabattier has well observed in treating on prolapses of the uterus and vagina). By this circumstance some have been deceived in believing such instances to be cases of Hermaphrodism.

49. An important question in the history of the vascular system of the mucous membranes presents itself, which is, does this system admit more or less blood, according to its various circumstances? As the organs within which this sort of membrane is spread are nearly all of them susceptible of contraction and dilatation, as is observable in the stomach, intestines, bladder, &c., it has been believed, that during their dilatation the vessels, being more spread out, received more blood, and that during their contraction, on the contrary, being folded on themselves, and as it were strangulated, they admitted but a small portion of this fluid, which then flows back into the adjacent organs. M. Chaussier has applied these principles to the stomach, the circulation of which he has considered as being alternately the inverse of that of the omentum, which receives, during the vacuity of that organ, the blood which it, being in a state of contraction, cannot admit. Since M. Lieutaud, an analogous use has been attributed to the spleen. Observe what I have ascertained on this subject from the inspection of animals opened during abstinence, and in the various periods of digestion.

50. (1) Whilst the stomach is in a state of repletion its vessels are more apparent on its exterior surface than during its vacuity; its mucous surface at this time has no higher degree of redness, but it has sometimes appeared to me to be less red than when the viscus was empty. (2) The omentum, being less extended during the plenitude of the stomach, presents nearly the same number of apparent vessels, equal in length, but more folded upon themselves than during the vacuity of that organ³. If they are then less loaded with blood the difference is scarcely perceptible. I would here observe, that great care is requisite in opening the animal, or the blood will fall upon the omentum, and prevent us from ascertaining its real state. (3) I am confident that there is no such constant relation between the volume of the spleen and the stomach in its different states of vacuity or plenitude; and if that organ increases and diminishes under various circumstances, it is not always in the inverse ratio of the state of the stomach. Like Lieutaud, I at first made experiments on dogs, in order to satisfy myself respecting the facts just stated; but the inequality in the size and age of those which were brought to me leading me to fear that I might not be able to compare their spleens correctly, I repeated them on Guinea pigs, whose size and condition corresponded, and examined, at the same time, some whilst the stomach was empty, and others whilst it was full. I have almost always found the volume of the spleen nearly equal, or at least the difference has not been very perceptible. Nevertheless, in other experiments I have seen the spleen, under various circumstances, to show variations in its volume, but more particularly in weight; and this was the same during digestion as after that process was finished. From what has been said it appears, that if, whilst the stomach is empty, there is a reflux of blood to the omentum and spleen, it is less than has been commonly asserted. Moreover, during this state of vacuity, the numerous folds of the mucous membrane of this viscus leaving it, as we have before said, almost as much extent of surface, and consequently of vessels, as during its plenitude, the blood must circulate there nearly as freely as when the viscus is in a contrary state; it has therefore no real obstacles; the only impediment is in consequence of the tortuous direction the vessels are then thrown into. Now this obstacle is easily surmounted, since the vessels suffer no constriction or diminution of calibre by the contraction of the stomach.

51. As respects the other hollow organs, it is difficult to examine the circulation of their adjacent viscera during their plenitude or vacuity; for their vessels are not superficial, as in the omentum, or insulated, as in the spleen; therefore, to decide this question concerning them, we can only observe the state of the mucous membranes upon their internal surface. Now they have always appeared to me as red during the contraction as during the dilatation of the organs. Finally, I give this only as a fact, without pretending to draw any inference from it opposed to the common opinion. It is, in fact, possible, that though the quantity of blood be always nearly the same, the rapidity of the circulation may increase; and consequently, in a given time, more of this fluid will be sent there during the plenitude of the viscera. This appears to be necessary for the secretion of the mucous fluids, which are then more abundant.

SECTION VI.
OF THE VARIATIONS IN THE ORGANIZATION OF MUCOUS MEMBRANES IN DIFFERENT REGIONS

52. The assemblage of the epidermis, corps papillaire, chorion, glands, and vessels, constitutes in the mucous membranes their intimate organization, which presents very considerable variations in the different regions in which they are examined. I shall point out only the principal of them; for in no different parts do these membranes present the same appearance, and in order to describe all their differences they should all be examined.

53. One of these variations is that which the aspect of mucous membranes presents at their origin, when compared with their appearance in the more remote parts of the organs. Compare, for instance, the surface of the glans, the inner surface of the lips, the orifice of the urethra, &c., with any portion of the inner surfaces of the stomach, intestines, &c. In the first the corps papillaire will be seen slightly marked, and offering no villous character, the epidermis thick, very distinct, and easily separated, the chorion very evident, the vessels rather less superficial, the mucous glands numerous and very large, more especially in the mouth; in the other characters almost opposite will be observed; we should say, that the mucous membranes have at their origin a structure of a middle kind between the skin and their deeper portions.

54. Another variation of structure, not less striking, is that which is met with in that portion of mucous surface which lines the sinuses. Here it has more redness, and an extreme tenuity; the three layers cannot be distinguished; and although there is a considerable secretion of mucous fluids, there are no perceptible mucous glands. Such are the characters of those portions of the pituitary membrane, which are considered as adapted to augment the sensation of smell, but which do not perform that function in the manner generally understood. In fact, the instant when an odour enters the nose, having the air for its vehicle, it cannot at once pass into the sinuses, because the orifices by which these cavities communicate with the nose are very small; but it enters gradually, impregnates all the air which they contain, and not being able to escape readily, for the same reason that rendered its entrance difficult, the sensation is prolonged, which on the general pituitary membrane is soon dissipated by the action of the fresh air. Thus therefore the pituitary membrane is destined to receive the impressions of odours, and its extensions into the cavities of the sinuses to retain them.

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.