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.