VII
VENUS'S FLY-TRAP.
1. Did you ever see a Venus's fly-trap? This curious plant grows in North Carolina. It is called a fly-trap because it has on each of its leaves something like a steel-trap, by means of which it catches flies. You can see one of these traps in the picture. When a fly touches the leaf, the trap shuts up at once, and the poor fly is caught and cannot get away. The harder it tries to escape, the more tightly the trap closes upon it, until after a time it is crushed to death.
2. But we have yet to learn the most curious thing about this strange plant, which seems to act so much like an animal. If we open the leaf after a few days, it will be found that the fly has almost entirely disappeared. The fly has not escaped, but it has been dissolved by a fluid formed inside of the trap, and the plant has absorbed a portion of the fly. In fact, it has really eaten it. The process by which food is dissolved and changed so that it can be absorbed and may nourish the body, is called digestion (di-ges´-tion).
THE DIGESTIVE TUBE.
3. The Venus's fly-trap has a very simple way of digesting its food. Its remarkable little trap serves it as a mouth to catch and hold its food, and as a stomach to digest it. The arrangement by which our food is digested is much less simple than this. Let us study the different parts by which this wonderful work is done.
4. The Digestive Tube.—The most important part of the work of digesting our food is done in a long tube within the body, called the digestive tube or canal.
5. This tube is twenty-five or thirty feet long in a full-grown man; but it is so coiled up and folded away that it occupies but little space. It begins at the mouth, and ends at the lower part of the trunk. The greater part of it is coiled up in the abdomen.
6. The Mouth.—The space between the upper and the lower jaw is called the mouth. The lips form the front part and the cheeks the sides. At the back part are three openings. One, the upper, leads into the nose. There are two lower openings. One of these leads into the stomach, and the other leads to the lungs. The back part of the mouth joins the two tubes which lead from the mouth to the lungs and the stomach, and is called the throat. The mouth contains the tongue and the teeth.
THE TEETH.
7. The Teeth.—The first teeth, those which come when we are small children, are called temporary or milk teeth. We lose these teeth as the jaws get larger and the second or permanent teeth take their place. There are twenty teeth in the first set, and thirty-two in the second. Very old persons sometimes have a third set of teeth.
SALIVARY GLANDS.
8. The Salivary (sal´-i-vary) Glands.—There are three pairs of salivary glands. They form a fluid called the saliva (sa-li´-va). It is this fluid which moistens the mouth at all times. When we eat or taste something which we like, the salivary glands make so much saliva that we sometimes say the mouth waters. One pair of the salivary glands is at the back part of the lower jaw, in front of the ears. The other two pairs of glands are placed at the under side of the mouth. The saliva produced by the salivary glands is sent into the mouth through little tubes called ducts.
9. The Gullet.—At the back part of the throat begins a narrow tube, which passes down to the stomach. This tube is about nine inches long. It is called the gullet, food-pipe, or œsophagus (e-soph´-a-gus).
10. The Stomach.—At the lower end of the œsophagus the digestive tube becomes enlarged, and has a shape somewhat like a pear. This is the stomach. In a full-grown person the stomach is sufficiently large to hold about three pints. At each end of the stomach is a narrow opening so arranged that it can be opened or tightly closed, as may be necessary. The upper opening allows the food to pass into the stomach, the lower one allows it to pass out into the intestines. This opening is called the pylorus (py-lo´-rus), or gate-keeper, because it closes so as to keep the food in the stomach until it is ready to pass out.
11. In the membrane which lines the stomach there are many little pocket-like glands, in which a fluid called the gastric juice is formed. This fluid is one of the most important of all the fluids formed in the digestive canal.
GASTRIC GLAND.
12. The Intestine(in-tes´-tine).—At the lower end of the stomach the digestive canal becomes narrow again. This narrow portion, called the intestine, is about twenty-five feet long in a grown person. The last few feet of the intestine is larger than the rest, and is called the colon. This long tube is coiled up and snugly packed away in the cavity of the abdomen. In the membrane lining the intestines are to be found little glands, which make a fluid called intestinal juice.
13. The Liver.—Close up under the ribs, on the right side of the body, is a large chocolate-colored organ, called the liver. The liver is about half as large as the head, and is shaped so as to fit snugly into its corner of the abdomen. The chief business of the liver is to make a fluid called bile, which is very necessary for the digestion of our food.
14. The bile is a bitter fluid of a golden-brown color. It is carried to the intestine by means of a little tube or duct, which enters the small intestine a few inches below the stomach. When the bile is made faster than it is needed for immediate use, it is stored up in a little pear-shaped sac called the gall-bladder, which hangs from the under side of the liver.
15. The liver is a very wonderful organ, and does many useful things besides making bile. It aids in various ways in digesting the food, and helps to keep the blood pure by removing from it harmful substances which are formed within the body.
16. The Pancreas(pan´-cre-as).—The pancreas is another large and very important gland which is found close to the stomach, lying just behind it in the abdominal cavity. The pancreas forms a fluid called the pancreatic juice, which enters the small intestine at nearly the same place as the bile.
17. The Spleen.—Close to the pancreas, at the left side of the body, is a dark, roundish organ about the size of the fist, called the spleen. It is not known that the spleen has much to do in the work of digestion, but it is so closely connected with the digestive organs that we need to know about it.
18. Please note that there are five important organs of digestion. The mouth, the stomach, the intestines, the pancreas, and the liver.
19. Also observe that there are five digestive fluids, saliva, gastric juice, bile, pancreatic juice, and intestinal juice.
1. The process of dissolving and changing the food so that it may be absorbed and may nourish the body is digestion.
2. The work of digestion is chiefly done in the digestive tube or canal, which is about thirty feet in length.
3. The mouth contains the teeth, and has three pairs of salivary glands connected with it, which make saliva.
4. The gullet leads from the mouth to the stomach.
5. The stomach is pear-shaped, and holds about three pints.
6. It has an upper and a lower opening, each of which is guarded by a muscle, which keeps its contents from escaping.
7. The lower opening of the stomach is called the pylorus.
8. The stomach forms the gastric juice.
9. The intestines are about twenty-five feet long. They form