The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

Everyman might very well dispense with such storage. But his body will insist upon hoarding droplets of fat in all sorts of odd corners—under his skin, in between his muscles, in his bone-marrow, in his liver, in the membranes of his abdomen—if he gives it achance. He must diet himself or take exercise to burn up his fuel surplus if he does not want to be encumbered with this provision against a highly improbable involuntary fast.

But to return to our story of digestion, the most important aspect of the stomachphase is the breaking down of proteins by pepsin and the consequent loss of structure which prepares the food for more effective action in the intestine.

The permeation of the food mass by gastric juice is a very slow process, so slow that the portion lying in the middle of the stomach may not be reached for over half an hour after swallowing. This fact is an important one from the point of view of starch digestion. Ptyalin cannot work in an acid medium and is therefore inactivated by gastric juice; but during the thirty minutes or so before the middle of the food mass is got at by the juice, digestion of starch by ptyalin continues. At the same time, there is another activity in the central zone. We invariably swallow with our food a certain number of bacteria; these organisms, finding themselves in a warm place with plenty of food material, grow and multiply exuberantly. There is time for at least one complete generation of bacteria before the acid reaches and exterminates the little colony.

As we have already pointed out, the process of gastric digestion is assisted by slow churning movements of the stomach wall. These movements are like the peristaltic waves that drive food along the oesophagus—they have the form of rings of contracted muscle, embracing the stomach and moving downward and to the right in the direction .of the duodenal opening. The waves move steadily and slowly. They originate near the middle of the stomach, at intervals of from fifteen to twenty seconds, and they creep along the stomach like ripples moying incredibly slowly over the surface of a pond. The effect of the waves is to mix up the stomach contents and to drive them towards the tube along which they are to depart.

* The clotting of milk which occurs in the stomach is due to a change in its protein constituents. This change is often taken as evidence for the existence of a separate enzyme, rennin ; but it is probably due to pepsin.

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THE SCIENCE OF LIFE

CHAPTER 2

The way out of the stomach, the opening of the duodenum, is called the pylorus (the gateway). It is guarded by a powerful ring of muscle which normally keeps the exit tightly closed. The pyloric muscle is a sentinel, and will not allow food to enter the intestine before it has been adequately treated in the stomach. During the early stages of gastric digestion, before the food mass is ready to pass on to the next stage, the pylorus remains shut; the sluggish contractions of the stomach cannot push matter through it, but merely produce eddies and currents in the food mass. As digestion proceeds the stomach-waves become more and more forcible, and the pylorus begins to open; with every wave a little jet of chyme escapes into the intestine, and so, when ready, it is passed on.

It is clear that the time spent by a meal in the stomach will depend upon the ease with which it is digested. Thus veal, particularly minced veal, is easily broken up and _ is not detained for long; but pork, being more resistant to the gastric juices, remains in the stomach for about four hours. If a draught of water be taken on an empty stomach, to quench thirst, the pylorus relaxes at once, and it reaches the duodenum in one or two minutes. For obvious reasons, the thoroughness of mastication will affect the digestion-time ; a properly chewed and shredded meal is much easier to permeate than a meal that is gulped down inlumps. That is why Mr. Everyman does well to read or talk during his meals and take and deal with his mouthfuls slowly. The cult of Fletcherists is a health cult which makes a great point of never swallowing solid lumps of food. Fletcherists just go on chewing until the food is practically liquefied. They claim that they are rewarded by great digestive tranquillity.

The small intestine is a tube about an inch across and some twenty feet long, which forms an intricate coiling mass in the abdomen. The whole tube is very uniform in its minute structure, but physiologically it is possible to distinguish between the first and second halves. In the former the digestive process is completed, while the latter is chiefly concerned with the absorption of food into the blood. We shall deal at present with the first half only.

We have seen how the intestine leads out of the stomach. Some four inches from the pyloric opening there opens into it a narrow tube. This tube has two branches coming from important digestive? glands, the liver and the pancreas, and it serves to