Before we quit this part of the subject it remains for us to say a few words respecting the class of substances which are properly termed medicaments. The medicaments are nearly related to the condiments in their action on the system, but with this difference, that they are not only disagreeable to the palate, but are, for the most part, incapable of being re conciled to it by habit. But there is in fact no exact line of demarcation between them ; many of the articles which are usually consi dered as condiments, being not unfrequently used in medicine, and some of what are gene rally regarded as the most active and nauseous medicines, being employed by some individuals as agreeable condiments. Both these classes of substances appear to differ in one essential particular from what are more properly re garded as articles of diet, that while it is essen tial to the operation of the latter, that they should be decomposed, and probably resolved into their constituent elements, the specific effect of the former seems to depend upon their acting on the stomach in their entire state. Nearly connected to this class of substances, and indeed differing from it only in degree, are the articles that are usually termed poisons. The term may, however, be regarded as entirely a popular designation, for as there is no active medicine which may not immediately destroy life by an excessive or improper administra tion, so there are no substances, among those which are usually considered as poisonous, which may not, under certain circumstances, prove valuable medical agents.
III. An account of the changes which the food experiences in the process of digestion.— We now proceed to the consideration of the third subject which we proposed for our in quiry, the nature of the change which the food undergoes during the process of digestion. In prosecuting this inquiry we shall consider in succession the various processes by which the aliment, after being received into the mouth, is brought into the state of chyle. These changes may be reduced essentially to three; the me chanical division of the food, as effected by the operations of maceration, mastication, and tri turation; the conversion of the alimentary mass into chyme, by the action of the gastric juice; and lastly, the conversion of chyme into chyle in the duodennin.* After the account which we have given above of the organs of mastication, nothing further remains for us to say on the first part of the process ; we may therefore conceive that the !bud, after it has been mechanically divided by means of the teeth or any analogous organ, is conveyed to the stomach, in order to be acted on by the gastric juice and converted into cliyine.1- The process of chymification consists in a certain chemical change, by which the aliment, from whatever source it may have been derived, and whatever may have been its origi nal constitution, is converted into a uniform pultaceous mass, having certain specific pro perties, which are different from those of the substances from which it is formed.
And we may here observe, that this kind of change, which has been frequently spoken of as something of a mysterious or inexplicable nature, is perfectly analogous to what takes place in all chemical action, where the addition of a new agent imparts new properties to the mixture. The supposed difficulty in this ease
has arisen from an indistinct conception in the minds of many physiologists, both of the nature of chemical action generally, and of the appro priate powers which belong to a living orga nized system. The essential and exclusive functions of vitality may probably be all re duced to two great principles of sensation and motion, as depending primarily upon the action of the nerves and the muscles. Chemical affi nity is independent of these principles, but it is, in various ways, modified by their operation, by bringing the agents into contact, by separa ting them from each other, and thus enabling them to produce new compounds, and when the compounds are formed, by removing them from the further action of the agents, and by conveying them to the situations when they are required, for the exercise of some new function. In the present case the glands of the stomach secrete a fluid possessed of specific properties; by the act of deglutition, and by the muscular contraction of the stomach itself, the alimentary mass is conveyed to the part where it may be brought into contact and mixed with this fluid. Each portion of the aliment is successively subjected to the due action of this agent, and when the process is completed, it is carried through the pylorus out of the stomach, while a new portion of aliment takes its place and goes through the same process.
In this part of our subject there are two points which will require our particular atten tion; first, we must ascertain the properties of chyme, and secondly, those of the gastric juice. It is commonly stated, that from whatever source the chyme is. derived, provided the stomach be in a healthy state, its properties are always the same,* and it must be admitted that, as a general principle, this would appear to be the case. In animals of the same species, notwithstanding the miscellaneous nature of thQ substances that are employed in diet, the result of the complete action of the stomach is a ina.ss of uniform consistence, in which the peculiar sensible properties of the articles of food cannot be recognized. But this statement must be re ceived with certain limitations, and is only ap plicable to the ordinary diet, for we have reason to believe, not only that the chyme produced from animal matter differs from that of vegetable origin, but even that different species of vege table aliment produce a different kind of chyme. The chyme from fruits or green vege table matter is notoriously more disposed to pass into the acetous fermentation than chyme formed from farina or gluten, a circumstance which must depend upon a difference in their chemical constitution. We also know that the same kind of aliment is differently acted on by the gastric juice of different individuals; but this may probably depend upon some variation in the nature of the gastric juice itself, and is therefore to be referred to a different principle.