If any thing could have opened Montègre’s eyes to the fallacy under which he laboured in considering the gastric juice as almost identical with saliva, the circumstance we are now to mention would have sufficed. When a person, previously in good health, dies by a violent death, or when an animal is killed soon after a meal, it very often happens that, on opening the body after an interval of some hours, the stomach is found to be eroded, and its contents poured into the cavity of the abdomen, precisely as if a hole had been formed in it by ulceration. It was long before the reason of this was discovered; but at length it was ascertained to arise from the action of gastric juice (the abundant secretion of which was provoked by the immediately preceding meal) upon the substance of the stomach, now subjected to its power from being deprived of life. This fact has been so often verified, that it is by all admitted as incontrovertibly true.[27] If, therefore, the gastric juice be merely saliva and mucus, we might expect to find after death distinct traces of similar results from the contact of saliva within the mouth or gullet; but there no such erosion is ever witnessed, nor, as Montègre himself admits, does saliva exert any solvent power whatever over dead animal matter out of the body. These facts appear quite sufficient to convince any unprejudiced mind.
The power of coagulating milk, and albumen or the white of eggs, is another remarkable property of the gastric juice,—and one so familiarly known, that in dairies an infusion of the stomach of the calf is in common use, under the name of runnet or rennet, for curdling milk. In infants, also, we know that the nurse’s milk has scarcely reached the stomach before coagulation takes place; a fact which leads many experienced mothers to infer that the infant is already suffering from acidity, and to counteract the supposed evils by repeated doses of magnesia—which, of course, do more harm than good. The coagulation of milk in the stomach, is so far from being a morbid process, that milk cannot be properly digested without it. By the separation and absorption of the fluid whey, the curd is reduced to a proper consistence for being acted upon, both by the gastric juice itself and by the contractions of the muscular coat.
The gastric juice is also powerfully antiseptic; that is to say, it prevents animal substances from becoming putrid, and even renders sweet such as have advanced a considerable way towards putrefaction. Dr Beaumont mentions that the pure juice will keep unchanged for almost any length of time, and according to Spallanzani meat may be preserved in it without taint for five or six weeks, or even longer. This antiseptic tendency of the gastric fluid, accounts for the circumstance that little or no mischief results from the common practice among epicures, of not making use of game till the putrefactive process is advanced farther than is agreeable to the palates of the uninitiated.
The qualities of the gastric juice are so directly adapted to the natural food of the animal, that flesh introduced into the stomach of an ox or a sheep, for example, undergoes scarcely any change; while vegetable food, on the other hand, remains equally undigested in that of a beast of prey. Thus, “when a hawk or an owl has swallowed a small bird, in the stomach of which have been seeds, these bodies are not dissolved by the gastric fluid,”[28] but pass through the intestines unaltered. Man, the dog, and some other creatures, possess the power of digesting all sorts of aliment, whether vegetable or animal, and are hence called omnivorous or all-eating; but even in them, the relation which the properties of the gastric juice bear to the qualities of the food chiefly or exclusively used is so close, that, when a widely different kind is suddenly resorted to, indigestion is the almost inevitable consequence,—because then the gastric juice has not had time to acquire its requisite adaptation to the new materials on which it has to act. Hence also the danger arising from suddenly eating a full meal after having been long famished. The stomach accustomed for a time to the smallest quantity, is no longer able to provide gastric juice sufficient for the purposes of digestion; and being irritated by continued craving, its secretions are vitiated and its sensibility exalted to such a degree as to require the wisest management to enable it to regain its healthy tone.
The gastric juice of carnivorous animals differs even in its chemical constitution from that of the herbivorous; a circumstance which accounts for the difference observed in their action. In the former, such as that of birds of prey, serpents, and fishes, no free or uncombined acid can be detected, although it is invariably found in the gastric fluid of vegetable-eaters. In crows and dogs, on the other hand, and such animals as can live on either kind of aliment, it is never acid except when they have been fed chiefly on grain or plants. In man, the same relation has, by numerous experiments, been ascertained to exist.
But although, in every class of living beings, the gastric juice is constituted with a direct relation to its natural food, still its qualities may be so much modified by a very gradual change of diet, as to fit it for digesting aliment of a very dissimilar or opposite kind. Thus, in the natural state, the stomach of a sheep exerts scarcely any action on beef or mutton; but if the change from the one kind of food to the other be made by slow degrees, the gastric juice will in the end become so essentially altered, as to enable it to digest both. In this way, as is mentioned by Delabere Blaine,[29] a horse at the Veterinary College was supported for some time by animal matter alone; while others have subsisted on dried fish, or on milk. It has been shewn, also, by John Hunter, Spallanzani, and others, that eagles, falcons, owls, pigeons, and domestic fowls, may for a time be fed on aliments altogether foreign to their natural habits. But these facts only chew the extent to which Nature will go, on an emergency, for the preservation of life; and no more indicate the equal fitness of both kinds of aliment, than the fact of some men being able to stand for a few minutes on their heads proves an inverted position to be the natural attitude of the human race.
In consequence of this adaptation of the gastric juice to the nature of the food, it is obvious that sudden and extreme changes from one kind of diet to another must be injurious, because the stomach has not time to modify its secretions sufficiently to meet the altered demand made upon its powers. This, accordingly, is one of the reasons why so much caution is used in bringing horses into condition after having been for some time in the pasture-field. When they have previously been on dry food in the straw-yard, corn may be given with greater safety: so that it is the change, not so much in quantity as in kind of aliment, which causes the risk. And, on this account, when a horse is to be put upon hard food, after having been fed on grass or other succulent vegetables, Blaine recommends, not only that hay and corn should be given in very small quantity at first, but that the hay should be moistened, and the corn mixed with bran and mashed; by which means having acquired a greater analogy to grass, it will be more easily acted upon by the gastric juice, which has been previously adapted for green food.
Even in man, the gastric juice undergoes considerable modifications, not merely according to the kind of aliment habitually used, but also according to the time of life, the wants of the system, the season of the year, and the state of the health; so that while sudden and great changes from one kind of diet to another are positively hurtful on the one hand, absolute uniformity is not less objectionable on the other, because it leads to so great a uniformity in the quality of the gastric juice habitually secreted as to render it incapable of acting with due effect on any accidental variety of food, to which a change of circumstances may compel us.
Many attempts have been made to ascertain to which of the elements of the gastric juice its power is chiefly to be ascribed, and experiments have been instituted on them individually to discover which of them is most nearly analogous to it in effect. From the general results, it appears that acetic acid (vinegar) and muriatic acid have a wider range of influence, and produce solvent effects more closely resembling those of gastric juice, than any other known substances. Both of these acids, it will be recollected, are constituent elements of the gastric fluid; and it has, in consequence, been argued, that to them it is indebted for all its energy. And, indeed, without laying too much stress on this real or supposed analogy, it is impossible to overlook the well-known fact, that scurvy, and a highly alkaline state of the system, are generally induced by a diet restricted for a long time to animal food alone, and are prevented or cured most easily by a free use of lemon juice or of vegetable matter, either fresh or fermented. In these circumstances, the vegetable acid is probably efficacious both by directly improving digestion, and by combining with the excess of the alkaline salts already existing in the system. It is worthy of remark, too, that in weak stomachs acidity is almost invariably induced by the use of vegetable food, possibly to some extent for the very purpose of effecting its digestion; for it has been ascertained beyond a doubt, that in herbivorous animals the gastric juice always contains some free or uncombined acid—and in man also, after living much on vegetables for some time.
The necessity of acid for the chymification of vegetable food, affords an explanation of the fondness which the Germans and Dutch display for saur-kraut—or cabbage in a state of acetous fermentation—and of its alleged easy digestibility. It explains, also, the general use of vinegar along with salads, cucumbers, oysters, salmon, and other substances of difficult digestion, and shews that its utility is not imaginary, but loudly proclaimed by Nature’s own acts.