The next theory which prevailed considered digestion to be the result of the putrefactive process. The single fact that the gastric juice not only arrests putrefaction, but even restores to sweetness meat in which that process is begun, is sufficient to demonstrate the wildness of such a supposition.
Another set of physiologists imagined that trituration would account best for all the changes occurring in the food during digestion; and consequently regarded the chyme as a sort of emulsion formed by the intimate mixture of the aliment with the juices of the stomach, just as an emulsion is formed by rubbing down almonds in a mortar. The advocates of this theory referred for proofs not only to the contractile motions of the stomach, already noticed, but to the muscular apparatus for trituration which forms so remarkable a feature in the gizzard of granivorous birds. But, in adopting this conclusion, they forgot that in birds the triturating apparatus does not digest, but serves, like the organs of mastication in man and quadrupeds, merely to bruise the grain on which the animal lives. In birds, in fact, digestion begins only after the trituration is finished.
A more recent and much more accurate view of digestion, is that which considers it as neither more nor less than a chemical solution of the food in the gastric juice. This theory is supported by a greater number of facts and experiments than any other; but although substantially correct, it is perhaps too exclusive and limited in its principles. It is true that, by the agency of gastric juice on food out of the body, a change very similar to chymification can be effected on it; but when we remember that chyme or the result of real digestion, is essentially the same in its elementary or component principles, whatever be the kind of food from which it is formed,—and that, as yet, we are acquainted with no purely chemical agent, which, applied to different substances, gives rise to the same uniform product,—we shall be more willing to believe that chymification is neither a purely mechanical nor a purely chemical operation; but the result of a vital process, to which both mechanical and chemical forces contribute, and which no action or combination of inanimate matter can either exactly imitate or supersede.
To enable ourselves to appreciate correctly the nature of digestion, we must begin by considering the conditions essential for its performance, or without which it cannot be carried on.
The FIRST indispensable requisite is an adequate supply of gastric juice, and its thorough admixture with every particle of the food on which it is to operate. The SECOND is a steady temperature of about 98° or 100° Fahr.; and the THIRD is the gentle and continued agitation of the alimentary mass in the stomach while digestion is going on.
In illustration of the influence of the FIRST condition. I may refer to the experiments already mentioned as having been made by Spallanzani, Stevens, and others, to shew the solvent power of the gastric juice on food even out of the body. Spallanzani states, that when small portions of well-masticated beef or mutton are placed in a vial with a due proportion of gastric juice, and the requisite temperature and gentle agitation are secured by placing the vial in the arm-pit, the appearances presented at the end of a few hours are extremely analogous to those observed in the natural process of chymification; the meat being in both cases converted into the soft greyish mass of a pultaceous consistence called chyme.
Dr Beaumont, who was well aware of the importance of Spallanzani’s researches, and of the almost universal adoption of his views by succeeding physiologists till confidence in their accuracy was for a time shaken by the bold and fallacious assertions of Montégre, felt that the opportunity afforded him by St Martin’s wound for verifying or disproving the experiments on which these views were founded, was much too valuable to be lost. He therefore entered upon a long series of investigations, of which the following is an imperfect, though I hope instructive, abstract.
To test the reality of the solvent powers ascribed to the gastric juice, Dr Beaumont withdrew from St Martin’s stomach about one ounce of it, obtained after a seventeen hours’ fast, by introducing first a thermometer to induce the secretion, and then a gum-elastic tube to carry it off. Into this quantity, placed in a vial, he introduced a piece of boiled recently-salted beef, weighing three drachms. He then corked the vial tightly, and immersed it in water raised to the temperature of 100°, which he had previously ascertained to be the heat of the stomach when the secretion was going on. In forty minutes, digestion had distinctly commenced on the surface of the beef. In fifty minutes, the fluid became quite opaque and cloudy, and the texture of the beef began to loosen and separate. In sixty minutes, chyme began to be formed. In one hour and a half, the muscular fibres hung loose and unconnected, and floated about in shreds. In three hours, they had diminished about one-half. In five hours, only a few remained undissolved. In seven hours, the muscular texture was no longer apparent; and in nine hours the solution was completed.
To compare the progress of digestion in the natural way with these results, Dr Beaumont, at the time of commencing the experiment just described, suspended a piece of the same beef, of equal weight and size, within the stomach by means of a string. At the end of the first half hour it presented the same appearances as the piece in the vial; but when Dr Beaumont drew out the string at the end of an hour and a half, the beef had been completely digested and disappeared, making a difference of result in point of time of nearly seven hours. In both, the solution began on the surface, and agitation accelerated its progress by removing the external coating of chyme as fast as it was formed. When the experiment was repeated with chicken instead of beef, the solution was slower, from the greater compactness of the chicken not allowing the gastric fluid to penetrate its substance so readily. Had the beef and chicken been masticated before being subjected to experiment, the difference between them in the rapidity of digestion would probably have been less.
To ascertain still more accurately the difference between natural and artificial digestion (the one in and the other out of the stomach), Dr Beaumont put twelve drachms of recently-salted boiled beef into a vial, with the same number of drachms of fresh gastric juice obtained through the opening of the stomach after a fast of eighteen hours; and then placed the vial in a basin of water on a sand-bath, where he kept it at the heat of 100° Fahr., and continued to agitate it gently. Digestion soon commenced and went on uniformly for about six hours, when it ceased. One-half of the meat was then dissolved, and the texture of the remainder loosened and tender,—resembling the same kind of aliment when ejected from the stomach partly digested some hours after a meal, as frequently seen in cases of indigestion. On weighing the undissolved portion which remained after all action had ceased, six drachms and twelve grains of the beef were found to have been digested by twelve drachms, or nearly double its weight, of gastric juice. It thus appears that a given quantity of gastric fluid can digest only a relative proportion of meat; so that, when more is eaten than what there is juice sufficient to dissolve, stomachic disorder must necessarily follow. In this latter case, Dr Beaumont found that the addition of fresh juice causes digestion to be resumed.