The Process of Digestion-the Digestive Apparatus-how Different Foods are Digested - the Importance Of Mastication

Now that we have considered the circulation of the blood and its purification in the lungs, we can consider the subject of digestion. Digestion is the process by which the food is prepared to be absorbed into the blood The food that we eat has to be changed and altered until it forms part of the blood itself. Now, anyone can see that cutlets, potatoes, and bread-and-butter in no way resemble the red fluid which we call the blood, and which constantly is circulating through our tissues, bathing them with nourishment. This food has first to be cut into minute particles by the teeth and stomach. Secondly, to be dissolved and made soluble, so that it can pass through the fine membranes lining the stomach and intestines directly into the blood-vessels. Some food, such as sugar, for example, can be dissolved and made ready to be absorbed into the blood without any chemical change taking place. Starch, on the other hand, is not soluble in water, and therefore must be altered chemically - changed from insoluble starch into soluble sugar. Fat is broken up into very minute particles, that is, it is " emulsified " before it is absorbed. Lean meat has to be broken up and altered by the digestive juices before it is taken up by the blood. In a later article we shall deal fully with the subject of diet. Here we need only say that foods are divided into five classes:

1. Carbo-hydrates - starches and sugars.

2. Fats: lard, butter, dripping, vegetable oils, etc.

3. Proteids: animal proteids, such as lean meat; and vegetable proteids, such as cheese, egg albumen, lentils.

4. Mineral salts: such as common table salt, and the salts in fruit juice.

5. Water.

Food first is taken into the mouth, where it is quickly churned by the muscles of the tongue and broken up by the teeth. At the same time it is acted upon by the digestive juice of the mouth, which is called the saliva. Then it is swallowed into the stomach, to be subjected to the movements of the stomach muscles and acted upon by the gastric juice. After a time it passes on to the intestines, where various juices fake their share in the process of digestion. Before going into this process in detail we must study the simple anatomy of the digestive system.

The Digestive Apparatus

The digestive apparatus consists of:

1. The mouth, with the teeth and the salivary glands.

2. The gullet, or oesophagus (a muscular tube passing from the mouth to the stomach).

3. The stomach, a pear-shaped organ lying to the left side, at the level of the waist, below the heart.

4. The small intestine, which is about 22 feet long, leading from the stomach. Into this opens the common bile duct, which receives bile from the liver and the pancreatic juice from the pancreas, two very important digestive juices. The small intestine lies in a convoluted mass and is continued on into the large intestine.

5. The large intestine. At the junction of the small and large intestines the vermiform appendix is attached, a little worm-like organ which is affected in the disease called appendicitis.

1 he junction of large and small intestines is low down on the right.side of the body. The large intestine passes upward in the right flank, then crosses the abdomen about the level of the waist, then downwards in the left flank. The last part of it is called the rectum, or bowel, and it is the terminus of the digestive canal. The large intestine is about five feet long, and is much wider than the small intestine.

Digestive System

The Process Of Digestion

By "digestion" the food is converted into a milky fluid, which can be taken up by the bloodvessels. The process is partly physical and partly chemical, and its aim is to reduce the food to a state of solution, so that it may pass through the thin walls of the vessels into the blood. The chief agents in dissolving the food are certain ferments which are found in the digestive juices, the saliva of the mouth, the gastric juice of the stomach, and the pancreatic juice of the small intestine.

We shall deal with these digestive juices in order, and with the foods they act upon. We have already said that the food, whilst being chewed in the mouth, is mixed with saliva, which, as well as being a digestive juice, keeps the mouth moist, and thus assists speech. It is secreted by certain glands called the salivary glands. About three and a half pints of saliva are poured into the mouth in twenty-four hours, and it is sometimes increased at the sight and smell of food, when the "mouth waters."

Medical there are three pairs of salivary glands - the parotid glands lie in front of the ears, the sublingual glands lie under the tongue, and the submaxillary glands under the lower jaw. In the disease called mumps it is these glands which are affected and become tender and swollen.

The saliva is a clear, colourless, alkaline fluid, and the invisible ferment it contains is called " ptyalin." This ptyalin has the power of chemically changing starch into sugar. If you chew white bread, which largely consists of starch, for long enough, a sweet taste is felt in the mouth because a certain amount of the starch has been changed into sugar.

These remarks ought to impress all who read them with the importance of chewing food. The longer the food is chewed, the more starch is changed into sugar. The people who bite and swallow their food without chewing sufficiently are not allowing time for digestion in the mouth. One result of this is that the stomach is overworked. It has to do the larger part of the work of the teeth. Another point is that whenever the mass of food passes into the stomach, the digestion of starch is stopped. The stomach does not act on starch, but deals with another part of the food - viz., the proteids.

Digestion In The Stomach

The food remains in the stomach about four hours, and all the time it is being rolled over and over and churned into minute particles, because the stomach is really a muscular bag. In the wall of the stomach are innumerable minute glands which secrete gastric juice. About fourteen pints of this juice are poured out daily by the glands into the stomach. This gastric, or peptic juice pours from these glands when the food reaches the stomach, and is mixed with the food, just as the saliva is in the mouth. Gastric juice is a clear, colourless fluid, slightly acid, because it contains a minute quantity of hydrochloric acid. It also contains a ferment called pepsin, which can act upon insoluble proteid matter (which lean meat, cheese, lentils, etc., largely consist of), and change it into a soluble proteid called peptone. This may be called the process of " peptonisation of the food."

If you were to put a little lean meat, cheese, and a spoonful of cooked lentils into a tumbler, and add to it either gastric juice extracted from the stomach of an animal, or simply water with a little hydrochloric acid and some pepsin, what would take place ? The meat, cheese, and lentils, which would be practically unaltered in ordinary water, would gradually be dissolved. That is what takes place in the stomach.

After a few hours' digestion, the food is broken up into a semi-solid substance called " chyme," which is simply food partly digested. The digestion of meat is carried on to a considerable extent in the stomach. Also the vegetable protcids, egg albumen, lentils, cheese,etc.

These protcids, or albuminous foods, are now converted into peptones, which are soluble in water, and rapidly pass through the thin membranous walls of the blood-vessels.

Whilst the stomach is churning and digesting the food, the openings at either end of the stomach are closed, but as soon as the chyme is formed the opening into the smaller intestine relax' The time occupied by digestion, of course, varies with the articles eaten. Fish and lightly cooked eggs take much less time to digest than roast beef or pork, but the average time for the digestion of a meal may be said to range from three to five hours.

Digestive power varies according to age and health. Well-cooked food is more easily digested. Warm food is more easily digested than cold food. A moderately sized meal is more quickly digested than a large meal. Thorough mastication hastens digestion.

Now we have considered (1) mouth digestion, when starchy foods are acted upon; (2) stomach digestion, when albuminous, or proteid foods are digested.

The other great group of food-stuffs which we have to consider are the fats. Fats undergo no change either in the mouth or stomach, but, as will be shown in the next article on this subject, they are emulsified in the intestines To be continued.