In treating of the properties of the gastric juice, I shall on all disputed points give a decided preference to the observations of Dr Beaumont over those of any other physiologist; because, although a few cases have occurred, in which, from external wounds, direct access has been given to the interior of the stomach, and Richerand and others have availed themselves of the opportunities thus afforded of adding to our knowledge of the digestive process, still, in most of them which have been recorded, the patients were a comparatively short time under observation, and were not sufficiently re-established in health to admit of either extensive or conclusive experiments being made. Whereas in the case which Dr Beaumont had the good fortune to meet with, the patient remained under his eye for several years, and in the enjoyment of the most robust health; so that ample time and opportunity were afforded for every variety of experiments which reflection could suggest, and for their subsequent repetition under such modifications as seemed to be requisite for clearly distinguishing the accidental from the constant and essential result. In addition to these reasons, it ought to be added, in justice to the American physiologist, that, from the excellent judgment with which he carried on his investigations, and the scrupulous care with which he announces his results and separates facts from theory, it is impossible not to place great confidence both in his personal qualifications as an observer, and in the general accuracy of his statements. Moreover, as he enjoyed the rare advantage of seeing what he describes to have taken place in the stomach during healthy digestion, his evidence comes before us with the strongest possible claims on our attention.[25]
The first disputed point which is conclusively settled by Dr Beaumont is, that the gastric juice does not continue to be secreted between the intervals of digestion, and does not accumulate to be ready for acting upon the next meal. By inducing St Martin to fast for some hours, and then placing him with the opening in the left side exposed to a strong light, so as to give a distinct view of the cavity of the stomach, Dr Beaumont found its only contents to consist of a little viscid and occasionally slightly acidulated mucus mixed with saliva, and in no instance did he perceive any accumulation of the proper gastric juice. The same results had indeed been obtained by Tiedemann and other physiologists before the publication of Dr Beaumont’s memoir; but the evidence of the latter is so much more direct and incontrovertible, that it may justly be regarded as setting the question for ever at rest.
Having proceeded so far, Dr Beaumont next endeavoured to discover at what time the gastric juice begins to be poured out, and under what conditions its secretion is carried on; and here again ocular inspection afforded him satisfactory results.
It has already been remarked, that, on pushing back the valve which filled up the opening into the stomach, the cavity within became visible to a considerable extent; and that when St Martin lay over for a time on the left side, a portion of the villous coat, large enough to exhibit several inches of its surface, generally protruded. Owing to these circumstances, Dr Beaumont could easily observe what changes occurred, both when food was swallowed in the usual way, and when it was introduced at the opening left by the wound. Accordingly, on examining the surface of the villous coat with a magnifying glass, he perceived an immediate change of appearance ensue whenever any aliment was brought into contact with it. The action of the neighbouring bloodvessels was instantly increased, and their branches dilated so as to admit the red blood much more freely than before. The colour of the membrane consequently changed from a pale pink to a deeper red, the vermicular or worm-like motions of the stomach became excited, and innumerable minute lucid points and very fine nervous and vascular papillæ could be seen arising from the villous coat, from which distilled a pure, colourless, and slightly viscid fluid, which collected in drops on the very point of the papillæ and trickled down the sides of the stomach till it mingled with the food. This afterwards proved to be the secretion peculiar to that organ, or, in other words, the true gastric juice; the mucous fluid secreted by the follicles, which some have mistaken for it, is not only more viscid, but wants altogether the acid character by which it is generally distinguished.
Pursuing his experiments, Dr Beaumont then found that the contact not only of food but of any mechanical irritant, such as the bulb of a thermometer, or other indigestible body, invariably gave rise to the exudation of the gastric fluid from these vascular papillæ; but that, in the latter cases, the secretion always ceased in a short time, as soon apparently as the organ could ascertain that the foreign body was one over which the gastric juice had no power. But the small quantity obtainable in this way is perhaps more pure and free from admixture, and therefore better adapted for examination, than any which can be procured under any other circumstances.
Various methods have been employed for procuring the gastric fluid in a state of purity. Pieces of dry sponge, inclosed in a small hollow perforated ball with a string attached to it, have been swallowed both by man and by inferior animals, and afterwards withdrawn to have the juice expressed from them. In some instances the stomachs of criminals and animals killed after fasting have been opened, and the secretion collected. At other times the juice has been procured by voluntary or artificial vomiting. None of these methods is equal to that employed by Dr Beaumont; but of the three the first is unquestionably the best, because, although no gastric juice previously exists, the very contact of the ball excites the secretion of a quantity sufficient to moisten the sponge. In the second mode of proceeding, any portion of juice secreted in consequence of a stimulus applied after the stomach is opened, must necessarily be very small and rendered impure by the large admixture of mucus which it will contain; while, by the third method, as the gastric juice does not exist ready made in the stomach, either none but merely mucus will be procured, or it will be expelled mixed with the food or substance which had previously elicited its secretion.
Gastric juice, in its purest form, and unmixed with any thing except the small portion of mucus from which it can never be obtained entirely free, is described by Dr Beaumont to be a clear transparent fluid, without smell, slightly saltish (probably from the admixture of mucus), and very perceptibly acid. Its taste, he says, resembles that of thin mucilaginous water, slightly acidulated with muriatic acid. It is readily diffusible in water, wine, or spirits, and effervesces slightly with alkalis—a direct proof of its acid nature. It coagulates albumen, and is powerfully antiseptic, checking the progress of putrefaction in meat. When pure it will keep for many months, but when diluted with saliva it becomes fetid in a few days. According to Professor Dunglison, to whom some was submitted by Dr Beaumont for analysis, it contains free muriatic and acetic acids,—phosphates and muriates with bases of potassa, soda, magnesia, and lime,—together with an animal matter soluble in cold but insoluble in hot water. Tiedemann and Gmelin, again, describe it as composed principally of muriatic and acetic acids, mucus, saliva, osmazome, muriate, and sulphate of soda, with little or no albumen; and, according to the same physiologists, the proportion of acid is always greatest when vegetables or other substances of difficult digestion constitute the chief part of the diet. Other chemists give an analysis somewhat different from either of these; a circumstance which was, indeed, to be expected, considering not only the differences caused by variations of diet and of health, but also the necessarily different degrees of purity of the fluid submitted to examination.
The most remarkable property of the gastric juice is unquestionably the power which it possesses of dissolving and reducing to the appearance of a soft thickish fluid mass every thing in the shape of food which is submitted to its action,—while it exerts no perceptible influence on living or inorganic matter; for, so far as is yet known, nothing which is not organized, or which is still alive, can serve as nutriment for the animal frame. Water is the only inorganic body which is taken into the system for its own sake, and all mineral and other inorganic productions enter it as component parts of previously organized substances of either an animal or a vegetable nature. To a great extent, indeed, vegetation seems to be merely a process for the conversion of inorganic matter into a proper nutriment for the support of animal life; and many species of animals seem in their turn to be little else than living machines for the conversion of vegetable substances into a nutriment fit for other species by which they are intended to be devoured. It is true that, in some parts of South America, the natives, pressed by want, consume quantities of a soft unctuous clay, which is of course destitute of organization; but as there is every reason to believe that no nourishment is derived from it, and that it merely serves to allay the pangs of hunger, such instances form no exception to the general rule.
It would have been easy for the Creator to bestow such a structure on all animals, as to make them subsist entirely on vegetable aliment. But the arrangement which He has seen fit to adopt, is the source of an infinitely greater amount of active enjoyment than what could otherwise have existed. Had there been no beasts of prey, the world would soon have been overrun with herbivorous creatures to such an extent, that their numbers would speedily have become excessive in reference to the possible supply of food, and there would have been infinitely more suffering from starvation and disease, than what actually arises out of their existing relation to each other. On the present plan, there is ample food and enjoyment for all; and when the time does arrive when one animal must become the prey of another, the deprivation of life is in most cases unforeseen, and the suffering which attends it is in general only momentary in duration. There is thus both complete enjoyment of life while it lasts, and a great additional field opened for the support of an immense class of animals, which, with their present constitution, could not otherwise have existed at all.[26]
The gastric juice, as already remarked, has no power over living animal matter,—a most wise and admirable provision, since otherwise it would at once have attacked and destroyed the very organ which produces it. This is the reason why certain worms are able to exist in the stomach of man and other animals; and if it were possible for an oyster swallowed directly from the shell to continue to live, it would also effectually resist every attempt at digestion. But it, in common with most other beings, soon perishes in circumstances so foreign to its habits; and when once dead, the gastric juice assumes the mastery, and speedily converts it into chyme.