英文科學(xué)讀本 第四冊(cè)·Lesson 27 Digestion

Lesson 27 Digestion

The body is a working machine, and every act of our daily life destroys some of its substance. The muscles as the movers of the body, the brain as the center of thought and intellect, the eyes, nose, ears—all parts of the body perform their work, but in the very act destroy some of their own substance. These tissues of the body must be renewed as they are destroyed, or the body would lose in weight and strength. It is the blood which does this work of building up what is worn away.

But whence does the blood get its materials for this work? The food which we take supplies the materials. The blood is actually made from the food which we eat. The food is changed into blood by the process of digestion.

Let us glance at this work of blood-making. The first part of the work takes place in the mouth. We call it chewing or mastication. You know that, as this work goes on, the food is moistened by a fluid in the mouth.

This fluid is called saliva, and is poured out from the lining membrane of the mouth. It has the power of changing starch into sugar. Starch, you know, is insoluble, but sugar is soluble. Bread and many of the things we eat contain starch. The saliva changes this starch into sugar; the sugar is soluble, and is easily absorbed into the blood.

When the food has been properly masticated, it is swallowed and carried by the gullet into a bag or pouch called the stomach. In this pouch it is rolled and churned about to still further break it up, and all the time a fluid called gastric juice is oozing out from the sides of the stomach itself to dissolve it. This gastric juice dissolves the lean meat and the glutinous parts of bread on which the saliva has had no effect.

The inner lining of the stomach is crowded with a network of blood capillaries, and these vessels suck up the fully dissolved matters of the food as the gastric juice continues to act upon them.

The stomach at its right-hand extremity becomes a narrow pipe, and forms the commencement of a long tube—the intestines. This tube if stretched out would measure five or six times the length of the person, but it is so folded and doubled upon itself that it is easily packed in the space provided for it below the stomach.

Resting on the intestines is a great organ, the liver. This organ is always at work preparing a fluid called bile, which is required for the complete digestion of the food. The bile, as it is prepared, passes into the tube of the intestines, and there mixes with the food that has come from the stomach.

Bile has the power of acting on the fatty parts of the food, and dissolving them, just as the saliva and gastric juice dissolve the other parts.

As the food passes along the intestinal canal, these dissolved fats are absorbed into the blood, and, with them, all that is of use. Only the useless, undigested remainder is discharged from the body.