As our knowledge of the nature of these processes was extended, and we were thus enabled to ascertain more correctly what was the change which was produced, our language became more correct and better defined, and the term fermentation was restricted to a spe cific operation, in which certain proximate principles, derived from organized bodies,f act upon each other, and enter into new elementary combinations. The process is generally pro moted by the addition of a substance called the ferment, which is employed to enable the bodies to act upon each in the first instance, although, when the action has commenced, its presence may be no longer necessary. The most familiar kind of fermentation is that by which a mixture of sugar and mucilage is con verted into alcohol, and that by which the same substances, when exposed to the atmos phere, and to a certain temperature, are con verted into acetous acid. flow far we are to extend the number of fermentations is a point respecting which chemists are not agreed, and indeed there appears to be no reason but that of convenience which can decide the point. We accordingly find that while Mr. Brande is disposed to restrict the term to the vinous and acetous fermentation,] others extend it to three, four, or with Dumash even to six processes.
Among these, one which is the subject of daily observation is the panary, or that by which dough is converted into bread, a change which appears to come strictly under the definition, as a spontaneous action among the elementary constituents of the body, by which a substance is produced, essentially different from the one from which it was composed. Now we are disposed to think that the same principle will apply to the conversion of aliment into chyme, and that it is little more than a difference in the mode of expression, whether we say that digestion depends upon chemical action gene rally, or upon that peculiar kind of chemical action which has been termed fermentation.
The foregoing remarks apply immediately to the production of chyme, and it still remains for us to consider whether the same mode of reasoning can he applied to the further conver sion of chyme into chyle. And it must be confessed that this part of our subject presents us with new difficulties, and that the analogy, which in the former case was imperfect, is apparently still more so, when we apply it to the action of chylification. Here we have a chemical change in the constituents, without the intervention of any assignable agent, at tended with the production of a new substance, in consequence, as far as we can judge, of the spontaneous action of the elements upon each other, and with the separation of the substance thus formed from the remainder of the mass. But although the operation may be somewhat more complicated, and although we may find it less easy to assign an efficient cause for each step of the process, there will be found nothing contrary to the recognized effects of chemical affinity. And with respect to the question,
how far these effects should be referred to the specific action of fermentation, we may remark that the result of the proper fermentative pro cesses is to form a new product, and to sepa rate the product thus formed from the residuary mass. Upon the whole therefore we may con clude, that although there are many points in the chemical theory of digestion that are still unexplained and require to be further investi gated, yet that we have no facts which directly oppose it, while :the difficulties which we feel on certain points would appear to be princi pally owing to the imperfect state of our know ledge on the subject.
V. Peculiar affections of the digestive or gans.—We now proceed, in the last place, to offer some remarks on certain affections of the stomach and its appendages, which are only indirectly connected with the function of diges tion. Of these the most important are hunger, thirst, and nausea; we shall consider in suc cession the causes of each of them, and the relation which they bear to the animal economy in general.
Ilunger is a peculiar perception experienced in the stomach, depending on the want of food. Its final cause is obvious, but respecting its efficient cause there has been considerable difference of opinion among physiologists, some referring it to a mechanical, others to a chemical action, while by a third set of writers it is referred exclusively to a peculiar Condition of the nervous system. Before we enter into the respective merits of these opinions it will be necessary to remark concerning the feeling excited by hunger, that it is one of a specific nature, as essentially different from the mere perception of touch, as the sense of sight is from that of mechanical pressure made on the ball of the eye. In physiological language the stomach may be regarded as one of the organs of sense, in the same way with the eye and the ear ; i. e. a part furnished with a spe cific apparatus for producing specific impres sions on a set of nerves appropriated to it, which convey to the mind certain perceptions, and which, by habit or by instinct, we connect with certain conditions of the organ. In most cases we are able to point out distinctly the nature of the agent which produces these per ceptions, as light when applied to the eye, and the undulations of the air to the ear ; in the particular case of the stomach we are not able to point out any corresponding agent of this description, and in so far the analogy between the stomach and the organs of sense must be considered as defective.