In studying digestion, it is well to keep in mind the twofold division of food into nitrogenous, or flesh-forming, and carbonaceous, or heat-producing, elements. The process of digestion differs with the character of the food. The purpose of digestion is to change and combine all the elements of food into a fluid which will mingle with the blood, become assimilated, and furnish nutriment for the body.
Digestion is not confined to the stomach. It begins with in-salivation in the mouth, and, after deglutition, is carried on in the stomach and intestines, the process continuing through the entire length of the alimentary canal, - a tube varying in diameter, and thirty-six feet in length.
The first process in digestion, as in any chemical analysis, is to crush the materials. The teeth are the grinders for reducing the food. If we bolt our food or swallow it in lumps, the soft coats of the stomach are made to do the tearing and grinding work of the teeth. A solvent being necessary, the saliva is secreted from the blood, and is poured through three pairs of glands into the mouth, each pair supplying a different saliva. This softens the starch and tender cellulose. Animals, like the beaver, which feed chiefly on woody matters, have very large salivary glands.
In health the saliva is always alkaline, especially during and after meals. It lubricates the mouth, and moistens the food so that it may assume a pasty condition. It is also necessary to the sense of taste, everything being tasteless that the saliva cannot dissolve. For this reason we cook and season our food so that it will excite the flow of the saliva.
"This saliva is poured into the mouth not to be cast out, but to do a specific work, then pass into the stomach and be again absorbed. If the system be drained of the saliva by profuse spitting, as is the case with those who use tobacco freely, the order of bodily functions is reversed, and the mouth is made to do the work of the kidneys, which is to carry away a large amount of the superfluous water and all the waste salts."
The saliva consists mostly of water, with a very small amount of saline matter and about five parts in one thousand of ptyalin, an albuminous ingredient, or ferment, which has the power of converting the starchy portions of food into sugar, and sugar into lactic acid, but does not act upon nitrogenous food. The saliva froths easily, and aids in carrying air into the stomach.
This is the first step in digestion, and the most important, as any error in the beginning leads to evil consequences which affect the whole process. It is also important, because it is wholly a voluntary process. While the food is in the mouth, we may masticate it thoroughly or imperfectly, and swallow or reject it. But when deglutition has carried it into the stomach, it is wholly beyond our control, and we are not responsible for the remainder of the process, only so far as it may be affected by error in the first stages. Bread, potatoes, and all starchy foods should be thoroughly masticated, and mingled with the saliva. Meat may be swallowed hastily, or knives may be made to do the work of the teeth in masticating animal food; but no chemist can prepare an artificial saliva to be mixed with starchy food, to save the trouble of chewing it. If a piece of dry light bread be masticated thoroughly, it will crumble and be quickly mixed with the saliva, and become sweeter the longer it is kept in the mouth. The alkaline saliva changes the starch into sugar, and begins the digestion. Try to chew a piece of hot bread and it at once assumes a pasty condition, which neither teeth nor saliva can penetrate, and is swallowed involuntarily. A piece of putty would not be more indigestible. If this experiment be tried, no other argument would be needed against the use of hot bread.
The stomach carries on the second part of digestion. The presence of food excites the flow of a fluid called the gastric juice, which is secreted in large quantities in the mucous membrane of the stomach. At the same time the muscular coats of the stomach contract, and produce a sort of churning motion, which carries the food round and round and over and over, exposing all parts of it to the action of the fluid. This gastric juice is always decidedly acid in its nature, containing hydrochloric acid. The lactic acid formed from the sugar in the mouth is also present in the stomach. This acid arrests the work begun in the mouth on the starchy foods, renders the alkali neutral, and acts only on albuminous food. It contains an albuminous ingredient, or ferment, called pepsin, in about the same proportion as the ptyalin of the saliva. The pepsin mixed with the lactic acid is powerful enough to dissolve all the albumen and fibrine of flesh food into albuminose, and sets free the starch, sugar, and fat, melts the fat, but does not change either. When there is an excess or deficiency of acid in the stomach, the digestion is abnormal. Pepsin is sometimes used as a remedy for dyspepsia, and is obtained from the stomachs of young, healthy pigs which are kept hungry.
As all food which is to nourish the system must be converted into a fluid form, any substances which are taken in fluid form and afterward solidified in the stomach, as blood or juice of flesh, milk, and raw eggs, must be changed again to a permanent liquid form before they can be absorbed. All nitrogenous matters are not only dissolved by the gastric fluid, but are modified so as to remain dissolved. These changed albuminous matters are called peptones. Oil plays an important part in these changes, so that, although oil is not digested, it serves a useful purpose in passing through the stomach.
The stomach would digest itself were it not protected by a sheathing of mucus and by a continual forming of cells called epithelium, during the process of digestion. The liquid or watery portions of food enter at once into the circulation by absorption. If too much water be taken with food, it dilutes the gastric juice and retards the digestion; as all that is not needed must be absorbed before digestion can go on.
Digestion is also retarded by the presence of very hot or very cold food, as everything taken into the stomach has to be changed to the normal temperature of 100. A large amount of food overloads the stomach, distends the muscular coats, and lessens the power of motion. Too little food is also a mistake, as the stomach needs a certain amount of bulk to work upon. Continual or irregular eating is wholly contrary to the intention of nature, since it does not allow the stomach time to rest and to form new cells to secrete digestive fluids. Flesh food that is finely minced, like hash, croquettes, and many entrees, passes rapidly through the stomach without being dissolved by the gastric juice; but when taken in larger pieces it remains long enough to be all digested.
The digestive power of the stomach is weakened when there is any undue action in any other part of the body, as in great muscular exertion or in powerful excitement of the brain. Therefore we should eat sparingly at first, when fatigued by exercise or study or when unduly excited, and should rest awhile after eating.
Lactic acid, small portions of sugar and digested nitrogenous substances, pass into the blood by absorption through the stomach veins. Thus the contents of the stomach leave it in two directions: a portion is absorbed through the coats of the stomach by the process of osmose, or the passage of fluids through animal membranes; the remainder passes through the pyloric opening into the duodenum and intestines for the completion of digestion. Food from the stomach enters the duodenum in an acid state, and in the intestines is mingled with three alkaline fluids, all containing soda.
The pancreatic fluid, secreted from the pancreas, digests the fatty matters. It breaks the large granules of oil and fat into a great many minute particles, and converts them into a milky liquid called chyle, which mixes freely with water and passes through the tissues of the intestines into the lacteals. It also changes the starch into sugar, and the sugar into lactic acid, but has very little action on albuminous substances.
The bile, secreted from the liver, plays an important part in intestinal digestion, the exact nature of which is unknown. Bile is a complex liquid, consisting of biliary acid in combination with soda. It certainly aids in the absorption of fat, and many suppose its purpose is to lubricate the walls of the intestinal canal; from its soapy consistency it effects a smooth, non-irritating passage of the contents. If there be any lack or surplus of the bile, it soon produces an injurious result in the system. The bile is in the intestines, and not in the stomach except when the action of the stomach is inverted, in nausea and vomiting; then the bile is forced up into the stomach instead of down into the intestines.
The intestinal juice, secreted in the mucous membrane the entire length of the intestine, combines the active and digestive powers of all the other secretions.
The lactic acid is formed so rapidly from the digestion of sugar that the contents of the intestine quickly becomes acidulous, and this completes the digestion of any portions of nitrogenous food not fully digested in the stomach. The combined amount of the salivary, gastric, pancreatic, biliary, and intes-tinal fluids secreted daily is twenty-one pints, of which the gastric juice forms more than one half. There are mechanical aids to intestinal as well as stomach digestion. The writhing, worm-like motion, or peristaltic movement, of the muscular coats of the intestines forces the food downward, and exposes all portions of it to the digestive fluids.
Notwithstanding all these powerful agents in digestion, a portion of useful matter passes through the intestines unchanged; and if there be a deficiency of either fluid, or a weakness of the muscular coats, or too great a quantity of irritating substance, like cellulose, woody fibre, bran, etc., the amount is increased. This is carried, with the innutritious portion, into the larger intestine, and forms a part of the excretions.