The hypothesis of trituration may be consi dered as having originated with the mechanical physiologists of the seventeenth century, and was apparently supported by the curious facts, which were, at that time, more particularly brought into view and minutely ascertained, of the great force exercised by the muscular sto machs of certain tribes of birds. The facts, although perhaps in some instances rather ex aggerated, were sufficiently curious, but the deductions from them were incorrect, first, in extending the analogy from one class of ani mals to other classes, where it was altogether inapplicable; and secondly, in conceiving of the trituration which takes place in these mus cular stomachs, as constituting the proper pro cess of digestion, whereas it is merely a preli minary process, equivalent to mastication. The aliment, after it leaves the gizzard, is in the same state of comminution into which it is re duced by the teeth of those animals that are provided with these organs, and is then sub jected to the action of the proper digestive stomach, and undergoes the process of chyrei fieation. On this point the experiments of Stevens and Spallanzani, which were referred to above, are quite decisive; they show clearly how far the agency of mechanical action is in strumental in the process of digestion, and they also show that some other principle is essentially necessary for its completion.* 1Vhile the mathematical physiologists were thus attempting to explain the theory of diges tion upon thp principles of mechanical action, their rivals the chemists, who in every point strenuously opposed them, brought forward their hypothesis of fermentation. This was originally, at least in modern times, advanced by Vanhelmoot, and was embraced by a large part of his coutemporaries and successors.t It may indeed be considered as having been for some time, the prevailing theory ; a circum stance which we may ascribe, partly to the comprehensive, or rather the indeterminate sense in which the term was employed, and partly from the actual phenomena attending the process, which were more easily referable to this operation than to any other which was then recognized.
The merits, or rather the truth of this hypo thesis rests, in some degree, upon the defini tion of the term fermentation, or the mode in which it was employed by the writers of that period. As far as we can understand their meaning, and perhaps we may even say, as far as they themselves attached any definite idea to their own expressions, they ascribed to this process every change which the constituents of the body undergo by their action upon each other. Fermentation was therefore the cause of the morbid changes which the system expe riences, as well as of its natural actions ; it was equally the cause of fever and inflammation, as of secretion and digestion ; and so far was this theory pushed, that even muscular contraction and nervous sensation were referred to certain fermentative processes. As our ideas on this subject became more correct, in consequence of the extension of our information, our language became more precise. The change which cer tain vegetable infusions undergo in the forma tion of alcohol was assumed as the type of this class of actions ; the controversy then took a new aspect, and the question at issue was, whether the change of aliment into chyme and afterwards into chyle ought to be referred to the same class of operations with that by which sugar and mucilage are converted into alcohol. This question we shall be more able to answer satisfactorily when we have taken a view of the next hypothesis, that of chemical solution.
The doctrine of chemical solution, as applied to the action of the stomach upon the aliment received into it, is, in many respects, very similar to that of fermentation, depending, as will be seen, partly upon the definition of the terms employed, and partly upon the minute obser vation of the various steps of the process. The
hypothesis owes its origin to the experiments of lteaumur, and was very much confirmed by those of Stevens and Spalianzani, so often referred to, and especially those of the latter experimen talist, where chymification was produced out of the body, simply by exposing the various species of aliment to the gastric juice obtained from the stomach, in a proper temperature, and under circumstances, as nearly as possible, re sembling those of the natural digestion.* Making a due allowance for the unavoidable causes of interference, the results may be regard ed as satisfactory, and they clearly prove one part of the hypothesis, that the vital operation of the stomach consists merely in providing the agent, and in bringing the alimentary substan ces within the sphere of its action. This con clusion is still farther sanctioned by the power of the gastric juice in suspending or correcting putrefaction, and in coagulating milk, both which properties are observed in experiments made out of the body, apparently in as great a degree as in the stomach itself, and which can only be referred to the chemical relations of the substances employed. These considerations must be allowed to he very favourable to the hypothesis of chemical solution, but still there are many very serious difficulties which we have to encounter, before we can regard it as fully established. Of these the most import ant is the objection, which has been frequently urged against it; and has perhaps never been satisfactorily repelled, that it is contrary to the ordinary operations of chemical action for the same agent to be able to reduce the various and heterogeneous matters that are taken into the stomach into a uniform and homogeneous mass, and this difficulty is further increased, when we perceive this powerful effect to be produced by a substance possessed of properties apparently so little active as the gastric juice.* These objections, and others of an analogous nature, have appeared to many of tho most emi nent modern physiologists to press so powerfully upon any hypothesis of digestion which is derived from either mechanical or chemical principles, that they have conceived it necessary to abandon altogether this mode of reasoning, and have referred it entirely to the direct action of what has been termed the vital principle. It is assumed that the internal coat of the stomach is endowed with a specific property, peculiar to itself, and essentially different from any merely physical agency, by which it acts upon the food and reduces it to the state of chyme. This vital property of the stomach is supposed to be proved, both by the necessity of having recourse to this kind of power, in consequence of the in adequacy of the ordinary properties of matter, and to be farther confirmed by certain facts that have been supposed to prove that the same substance is differently affected by the gastric juice, merely in consequence of the absence or presence of this principle. Thus it has been observed, that in cases of sudden death, the stomach itself has been partially digested by the gastric juice that was secreted during life,t and upon this principle it has been found, that cer tain kinds of worms, which exist in the diges tive organs of animals, are not affected by the gastric juice as long as they remain alive, but that after death they become subject to its action.