DIGESTION. (Fr. digestion; Germ. Ver danung ; 1 tal. digestione.) This term is em ployed in Physiology to designate that func tion by which alimentary matter is received into an appropriate organ, or set of organs, and where it is subjected to a specific action, which adapts it for the purpose of nutrition.' In its original and technical sense this action was confined to the stomach,t but it is gene rally applied more extensively, so as to include a number of distinct operations, and a suc cession of changes, which the food experiences, after it has been received into the stomach, until a portion of its elements are separated from the mass, and are conveyed, by means of the lactcals, to the bloodvessels.
In the following article we shall employ the term in its most extensive acceptation, and shall regard the whole as one function, the successive steps of which are intimately and necessarily connected together, and each of them essential to the completion of the whole.r We shall commence by a description of the organs of digestion, we shall next give an ac count of the nature of the substances usually employed as food ; in the third place we shall trace the successive changes which the food experiences in the different parts of the pro cess; in the fourth place we shall examine some of the hypotheses that have been pro posed to explain these various operations, and shall conclude by some remarks on certain affections of the digestive organs, which are connected with, or dependant upon, their functions.
I. Description of the organs qf digestion.— The organs of digestion, taken in their most comprehensive sense, may be arranged under three divisions: the first, by which the aliment is prepared for the chemical change which it is afterwards to experience, and is conveyed into the stomach, being principally of a mechanical nature ; secondly, what have been more ex clusively termed the proper digestive organs, where the aliment receives its appropriate chemical changes ; and lastly, those organs by which, after the nutritive substance thus elaborated has been separated from the mass, in order to be conveyed into the blood, the residuary matter is expelled from lie system.§ In the higher orders of animals, where the functions are more numerous, and more varied in their nature, we find them to be so inti mately connected together, and dependent on each other, that it is impossible for any one of them to be suspended without the derange ment of the whole. But as we descend to animals of a less perfect and complicated structure, the functions are considerably re duced in number, and seem also to be less intimately connected, so that certain of them are either altogether wanting, or are performed, although imperfectly, by other organs, which are not exclusively appropriated to them.
Thus we observe that some, even of the parts which are the most essential to human ex istence, as the brain, the heart, and the lungs, are not to be found in many very extensive classes of animals, some of the functions be longing to these organs being entirely deficient, or being effected in a more simple or a less complete manner, by a less complicated ap paratus. As we descend still lower in the scale, we find the functions still more restricted and simplified, until we arrive at the lowest term which would appear to be compatible with the existence of an organized being, where no functions remain but those which seem to be essential to the original formation of the animal and to its subsequent nutrition. That some apparatus of this description is abso lutely essential may be concluded, both from the consideration, that the nutritive matter which is received into the system must un dergo a certain change, either chemical or mechanical, before it can be employed for this purpose, as well as from the fact, that a sto mach, or something equivalent to it, has been found to be the circumstance, which is the most characteristic of animal, as distinguished from vegetable life.* Accordingly, with a very few exceptions, and those perhaps depending rather upon the inaccuracy of our observation, than upon the actual fact, it is generally ad mitted, that every animal, the size and texture of which admit of its being distinctly ex amined, is possessed of some organ appro priated to the purposes of digestion.} Of the three orders of parts mentioned above, the second is the only indispensable one, or that which is alone essential to the due per formance of the function. In many cases the aliment is directly received into the stomach, without any previous preparation, either che mical or mechanical, and there are not a few instances in which the residuary matter is im mediately rejected from the stomach, without any distinct apparatus for its removal. In the following pages our main object will be to give an account of the function of digestion as it is exercised in man and in those animals which the most nearly resemble him, referring to other animals only so far as it may contribute to illustrate or explain the nature of the ope ration in the human species.