CHAPTER XXX. WHAT, HOW, AND WHY WE DIGEST. What we digest.-Staple elements of food, whether animal or vegetable.-How we digest.-What takes place in the mouth.-The saliva; quantity discharged into the mouth; its composition and functions--Properties of ptyalin.-The saliva ig alkaline; always on the watch for the entrance of food into the stomach.-Structure of the alimentary canal.-The stomach and its appendages.-What takes place in the stomach.-The starch, fat, and gluten, are brought into a liquid state.—Dissolving action of the pepsin.-Absorption from the stomach itself.-What takes place below the stomach.-Introduction of liquids from the gall-bladder and pancreas.-Supposed action of the bile.-Properties and uses of the pancreatic juice.Intestinal juice or mucus.-The universal solvent.-Absorption by the lacteals.— Changes of the chyle in the lacteals--Mesenteric glands.-Absorption by the veins. -Digestion in the large intestines.-Acidity in the cœcum.-Final discharge of the food from the intestines.-Why we digest-it is to form blood.-Purposes served by the blood.-Composition of the whole man, and of his blood.-Bodily functions discharged through the aid of the blood.-Bodily waste and motion connected. Special provisions for digestion in carnivorous and herbivorous races.Digestion in the sheep.-Purpose of digestion the same in all animals. WHAT We digest, how we digest, why we digest-how wide and interesting a field is embraced by these three topics! I. WHAT WE DIGEST.-This topic has already been sufficiently dwelt upon in considering the bread we eat and the beef we cook. Whether we sustain ourselves by means of vegetable or of animal food, we introduce nearly the same substances into the stomach. These different forms of food consist respectively The bread-of gluten, starch or fat, and saline matter. And, as we have seen, gluten and fibrin on the one hand, and starch and fat on the other, serve similar purposes, and may take the place of each other almost indifferently in a nutritious food. These, therefore, along with the saline matters cóntained in both animal and vegetable food, are the main substances we digest. It is true that vegetable food contains insoluble woody fibre in considerable proportion. In the bran of the bread we eat, and in the green vegetables and potatoes we consume, it is present in notable quantity; and it forms a very large part of the hay and other dried vegetable food with which cattle are fed. This woody fibre, however, passes through the animal, for the most part, useless and undigested. The digestive organs extract, from among the useless materials which the food may contain, the three staple forms of matter above described. We have only to follow these substances into the body, therefore, and see what becomes of them. II. HOW WE DIGEST.-The process of digestion involves three successive series of operations, mechanical and chemical. The first of these takes place in the mouth, the second in the stomach, and the third in the intestines. pre 1°. What takes place in the mouth.-We have already seen that in ripe fruits and other kinds of vegetable food pared by nature for immediate eating, the solid nutritious matter they contain is very minutely divided, and is intermixed with a large proportion of water. We have seen, also, that the first object of the cook, in a great number of our ordinary culinary operations, is to bring the raw food into the same minutely divided and highly diluted condition. But all the food we eat is not so prepared, either by nature or by art. The first operation we perform upon it, therefore, WHAT TAKES PLACE IN THE MOUTH. 295 is to grind it, if necessary, by means of the teeth, and to dilute and season it by means of the warm, fluid, saltcontaining saliva. It is then swallowed, and allowed to descend to the stomach. This operation appears to be altogether mechanical; and yet the chemical history of the saliva, which takes so great a part in the operation, and the relations of this saliva to the food, are both interesting and important. The saliva is secreted in glands which open into the interior of the mouth (fig. 105), and which, in some animals, are of large size. The quantity of liquid which these glands discharge into the mouth, and thence into the stomach, is very variable. In the case of the full-grown man it is sometimes as low as eight and sometimes as high as twenty-one ounces in the twenty-four hours. The saliva consists for the most part of water, and therefore, as I have said, its first function is to dilute the food. But this water holds in solution about one per cent. of saline matter; so that, to a certain extent, it may be said also to season the food. In the twenty-one ounces sometimes swallowed in a day, there are about eighty grains of this saline matter. The seasoning this gives to the food not only renders it more grateful to the palate, but prepares it also for the after changes it is to undergo in the stomach, and the uses it is to serve in the body. That this saline matter, though small in quantity, really does produce some beneficial effect upon the food, is rendered more probable by the influence generally ascribed to another. substance which is contained in the saliva in still smaller quantity. This substance is a peculiar organic compound, to which, from its occurring only in the saliva, the name of ptyalin is given. Like the diastase described in a previous chapter, ptyalin possesses the property of changing the starch of the food into sugar. This property it exhibits, according to some, when used alone-according to others. only when mixed with the saline constituents of the saliva. It forms less than one five-hundredth part of the whole weight of the saliva. Not more, therefore, than from fifteen to twenty grains of it are swallowed by a healthy man in the twenty-four hours; yet this small quantity is really of much consequence to the easy and comfortable digestion of the food. Hence it is that experience has recommended to all good livers a careful mastication of their food, that all parts of it may be thoroughly mixed with the saliva, and thus subjected to its chemical action. Two other facts regarding the saliva are of much interest as wonders of the human frame, independent altogether of their intimate relation to the process of digestion. One of these is, that the saliva has generally an alkaline* character —that this alcalinity is greater during and immediately after eating, and gradually lessens, till after long fasting the saliva becomes acid-that it is greater, also, after substances have been eaten which are difficult of digestion-and that, when the saliva discharged into the mouth is spat out instead of being swallowed, acidity and heartburn often ensue. (WRIGHT). these circumstances argue not only a close connection between the process of digestion and the alkaline character of the saliva, but an immediate watchfulness, as it were, over the immediate wants of a particular bodily organ. The other fact is, that as soon as food is swallowed, the saliva begins to flow more copiously than before. This is the case even if the food be swallowed without chewing. Or if food be introduced by an artifical opening into the stomach, without passing through the mouth at all, the saliva will forthwith begin to discharge itself into the mouth, with its *Substances are alkaline which have the taste of pearl-ash or common soda, or which restore the colour of vegetable blues that have been reddened by an acid. FLOW OF THE SALIVA. 297 alkaline character, and hasten down the throat to assist in the digestion. It appears strictly correct to say that the saliva is constantly on the watch to be useful, when we recollect how the mouth will often "water" at the mere mention of savoury articles of diet. When chewed and duly thinned with saliva, the food is rolled into a ball by the tongue, and is swallowed or forced the gullet, is an oblong rounded bag, capable, when moderately distended, of containing two or three pints. The annexed fig., 106, shows the form of the human stomach, and of the neighbouring organs which are concerned in the process of digestion. It exhibits, also, their relative positions. and their comparative sizes. The parts, as here shown, are a little distorted, from the necessity of turning up the liver VOL. II.-13* |