Those who contend that digestion is a simple solution, have sought, with some anxiety, for a solvent of a peculiar power; and they have at last, apparently, discovered it in what they style the gastric juice, a mucous fluid always found in the stomach, of some peculiar, but no very decisive properties. It is, undoubtedly, a solvent out of the body; but the experiments made by forcing animals to disgorge what they have taken, prove the power of all the fluids of the stomach, not of one only. In every part of the human body, the production of a fluid of peculiar powers is connected with a complicated apparatus. In the stomach the mucous glands only seem to produce the gastric juice. When analysed, it is said, by Struve, to contain a phosphorated ammonia; but this conclusion has not been supported by other chemists. Carminiati digested some veal in water, with a little salt, in a heat of about 100 of Fahrenheit. The decanted liquor he employed in a similar experiment, which he repeated till he produced a fluid resembling the gastric. This would lead us to a suspicion that the gastric juice is only the remnant of former digestions, and, in reality, nothing more than a ferment. We are told, however, by Carminiati himself
(Journal de Physique, vol. xxi.), that the gastric fluid of herbivorous and of carnivorous animals, when the stomach is not organically diseased, assists digestion, and cures intermittents, if given as a medicine. Other authors have told us, that it is antiseptic; that it even sweetens putrid meat; and that it greatly assists in healing old ulcers. It is natural to suspect the existence of extraordinary qualities, when detailed by those who, previous to their experiments, had formed a particular system. We may just suggest, however, that if the gastric juice has such miraculous powers, why does it not constantly produce them in the stomach ? If, after a meal made with the best appetite, a cause of sud-ded agitation or of deep distress should occur, the food, which would have otherwise produced wholesome chyle, becomes acid or putrid.. The stomach is not, therefore, a containing vessel only, gently agitating the mass.
Fordyce, after describing the structure of the organs of digestion; the matters applied to the food in those organs; and pointing out that the substances employed for food have the same elements, and each of them all the elements, actually found in chyle, viz. a part which is fluid, and contained in the lacteals, but coagulates on extravasation:- a second, which consists of a fluid co-agulable by heat, and, in all its properties that have been observed, consonant to the serum of blood; and a third, formed of globules, which render the whole white and opaque; observe, that it was, therefore, only necessary that these elements should be separated from one another, and recombined in order for its formation. That the action of the organs of digestion disunited the ele-mentsof the food, which were reunited in a new form, so as to form the essential parts of the chyle; and that these three essential parts of the chyle were always the same; and, therefore, when converted into blood, the blood a fortiori, could not, in the smallest degree, be influenced by the food. The elements, according to this author, are separated in the stomach, where they are retained; but the chyle is not formed in this organ. A simple matter, called chyme, is only there produced, which in the subsequent state of the process is, by the reunion of its elements, formed into chyle.
Digestion consists of two distinct stages: the first, which takes place in the stomach, styled solution, or fermentation; and the second in the duodenum, which is more strictly animalisation. In the remaining track of the intestine, the animalisation is rendered more complete: the chyle, gradually and progressively formed, is absorbed, carried into the blood, and then applied to supply the different organs, after having beenmore completely elaborated in the lungs. We must pursue this progress in the order.
It has been always, but with little accuracy, supposed that digestion takes place in the stomach. This is not true; for not a particle of chyle can be at any time discovered in its lymphatics, coloured matters do not tinge their contents; and what is called the chyme resembles in no respect the white fluid destined, at a future period, to fill the blood vessels. In the stomach then, perhaps, we shall find solution only in the saliva and other glands of that organ: but it is a solution which we may try in vain to imitate; because it takes place in an animated organ, whose power cannot be for a moment intermitted, without some injury to the result. Fermentation, also, probably soon comes on; and it is apparently assisted by the gastric juice, which acts as a leaven, and in this way only. Such a process is peculiarly necessary in herbivorous and granivorous animals, since the change by solution alone is not sufficiently rapid for the wants of the system, or equal to the effect of breaking down the denser substances; and since, perhaps, one stage of assimilation may be necessary in this organ to prepare it for the second. If the gastric juice be formed, as Carminiati suspects, and which is highly probable, since it differs in different animals, and always resembles the nature of their food, it is a ferment peculiarly adapted, not only to assist the separation of the parts, but, in some degree, to assimilate them to its own nature. The fluids of the stomach have no effect in dissolving the husks of vegetables, for instance; and consequently fermentation, in some cases, is absolutely-necessary. So far, therefore, there is little mystery.
The distention of the stomach, after a full meal, is not, in the greatest degree, at its termination, if the appetite has not been stimulated by varieties, or by condiments. It is farther increased by the separation of air, which is sometimes so considerable as to rise to the cardia, and be discharged through the oesophagus. It is, however, generally again absorbed by the mass, and carried into the blood. The food detained in the stomach by the action of its longitudinal fibres is lessened in its bulk by the reunion of its air, and the absorption of its watery parts. The pylorus is then brought in a straighter line with the axis of the organ, and the fluid parts gradually pass over. It has been supposed that the pylorus possesses an elective power, to admit of the passage of some bodies, and to refuse others. In every part of the animal economy we seem to perceive a similar power; nor dare we deny it to an organ of so much importance, whose nerves are peculiarly and acutely sensible. Richerand, who has most pointedly enforced this opinion, seems to think that this elective power is at last lost. The stimulus which excited, by its disagree-ble impression, the contraction of this aperture, becomes, in time, habitual, and the passage is effected. Independent, however, of this cause, heavy bodies not soluble in the watery fluids are not, for a long time, discharged. They lie at the bottom of the stomach; are not presented to the pylorus when it is full; and, when empty, they cannot rise to the angle which that passage forms. In this way, heavy metallic bodies, and even the heavy mucus of weak stomachs, are not discharged. The stimulus of the latter soon becomes habitual; but, after some time, the action of this organ seems to be excited, and the former are propelled into the duodenum.