This hypothesis of the vital principle is the one which was supported by Fordyce in his elaborate treatise, and is probably that which, under certain modifications, may be regarded as the prevailing opinion of the modern physiolo gists. To a certain extent it is correct, and the position on which it is founded, that the living body differs essentially in its powers and pro perties from the dead body, cannot be denied. But it may still be questioned, whether the ex planation thus offered be not rather verbal than real, or whether any actual explanation is afforded of the phenomena, or any actual diffi culty removed by adopting this mode of ex pression. Every one admits that a living sto mach differs from one that is deprived of life, but still it remains for us to point out in what this difference consists ; is it a chemical or a mechanical action ? or if it be not referable to either of these actions, to what general principle can it be referred ? It is contrary to the rules of sound reasoning to invent a new agent for the urgency of the individual case, until we are able to demonstrate the absolute impossibility of employing those which were previously recognized. With respect therefore to the hypothesis of the vital principle, as maintained by Fordyce and many of the modern physiolo gists, we should say, that it is rather a verbal than a real explanation of the phenomena, and that it rather evades the objections than answers them.
The last hypothesis of digestion which we proposed to notice, that of nervous action, although somewhat allied to the one which we have last examined, is more precise and defi nite in its statement, and consequently more entitled to our consideration. It assumes, that the process of digestion depends upon the direct and immediate agency of the nervous system. It is founded upon the anatomical fact of the mode in which the stomach is con nected with the nervous system, and upon the observed relations between those causes that act through the medium of this system, and the changes that take place in the action of the stomach. With respect to the anatomical ar gument it has been urged, that there is no organ of the body, which is provided with such a number of nerves, proceeding from so many sources, and connected in so direct a way with the cerebral system. There are equally remarkable circumstances of a physiological and pathological nature, which prove the inti mate connection between the nervous system and the action of the stomach. Not only does the stomach partake of almost every change that occurs, in any part of the corporeal frame, either natural or morbid, in a way which we must conceive can only be brought about through the intervention of the nervous sys tem, but it is affected by our mental emo tions, and that probably in a greater degree than any other of our organs, except those that, are immediately connected with the external senses. Its functions arc excited or depressed by various causes, which can only act through the medium of the mind or imagination; while it is argued that in all cases its various condi tions and the changes which its functions expe rience can be referred to no cause, except to corresponding changes in the nervous This hypothesis, like that of the vital prin ciple, has been supported by the consideration of the inadequacy of all the other modes of explaining the phenomena, and the impossi bility of referring them either to mechanical or to chemical principles. But it has this clear and decided advantage, that it rests upon the co-operation of an actual agent of great and acknowledged power, one the existence of which is universally recognized, the only ques tion being whether it is applicable to this indi vidual case. But although we admit the facts
in their full force, we must still demur to the conclusions that must be deduced from them. If we inquire upon what principle, or by what medium the nervous system can operate on the digestive functions, two modes present them selves to the mind. We may ascribe the effect either to the general operation of the nervous energy, whatever this may be, which pervades every part of the system, and the stomach among the rest, and which gives it those powers which distinguish living from dead matter; or we may conceive that the ner vous system is, in some way, more especially concerned in the production of the gastric juice, and that consequently whatever tends to decrease or diminish the nervous energy, may operate in the increased or diminished produc tion of this secretion, and thus indirectly, al though necessarily, affect the digestive func tion. But although we may admit the truth of both these suppositions, we gain no specific answer to our inquiry. It is not enough to be informed that the stomach acts upon its con tents because it is alive, or that whatever pre vents the secretion of the gastric juice puts a stop to the digestion. Our inquiry embraces a farther object, and leads us to investigate the nature of the connexion between these facts and the ultimate effect produced, or to discover the reason why certain acknowledged effects are connected with certain acknowledged causes; but to this question the nervous hypothesis gives us no satisfactory answer. It indeed rather involves the theory of secretion than of digestion, for even were it to be clearly proved that the nervous power (whether, according to the hypothesis of Dr. Philip, we identify it with the galvanic influence, or we act the more cautious pan of not attempting to explain its i nature,) is the immediate agent in the tion of the secretions, still we are left equally uninformed concerning the mode in which this fluid, when secreted, performs its appropriate function.• From this brief review of the different the ories of digestion we may conclude, that the hypothesis of trituration is decidedly incorrect, and that those of the vital principle and the nervous energy do not resolve the question. V e are therefore reduced to the two chemical hypotheses, which, although not without con siderable difficulties, are not so palpably defec tive or erroneous. In deciding between these two hypotheses it must be our first object to ascertain the exact sense in which the term fermentation was used by the older physio logists, and how far, according to the modern use of the term, it is applicable to the phe nomena in question. The word was originally employed in a very extensive, and, as may be supposed, in a somewhat vague manner, to designate every spontaneous change which took place between bodies that were placed in con tact, and which generally manifested itself by the extrication of some gaseous or volatile matter. Thus all the spontaneous changes in the body, whether natural or morbid, were considered to be different kinds of fermen tations, and many of the changes that take place among inorganic substances, as well as various processes in the laboratory, were dis tinguished by the same appellation.