~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 ca.n.a.l.
[Ill.u.s.tration: GASTRIC GLAND.]
~12. The Intestine~(in-tes"-tine).--At the lower end of the stomach the digestive ca.n.a.l 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.
SUMMARY.
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 ca.n.a.l, 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 the intestinal juice.
10. The liver lies under the ribs of the right side. It is about half as large as the head. It makes bile.
11. When not needed for immediate use, the bile is stored up in a sac called the gall-bladder.
12. The pancreas is a gland which lies just back of the stomach. It makes pancreatic juice.
13. The spleen is found near the pancreas.
14. There are five important digestive organs--the mouth, the stomach, the intestines, the liver, and the pancreas.
15. There are five digestive fluids--saliva, gastric juice, intestinal juice, bile, and pancreatic juice.
CHAPTER VIII.
DIGESTION OF A MOUTHFUL OF BREAD.
~1.~ Let us suppose that we have eaten a mouthful of bread, and can watch it as it goes through all the different processes of digestion.
~2. Mastication.~--First, we chew or masticate the food with the teeth.
We use the tongue to move the food from one side of the mouth to the other, and to keep the food between the teeth.
~3. Mouth Digestion.~--While the bread is being chewed, the saliva is mixed with it and acts upon it. The saliva moistens and softens the food so that it can be easily swallowed and readily acted upon by the other digestive juices. You have noticed that if you chew a bit of hard bread a few minutes it becomes sweet. This is because the saliva changes some of the starch of the food into sugar.
~4.~ After we have chewed the food, we swallow it, and it pa.s.ses down through the oesophagus into the stomach.
~5. Stomach Digestion.~--As soon as the morsel of food enters the stomach, the gastric juice begins to flow out of the little glands in which it is formed. This mingles with the food and digests another portion which the saliva has not acted upon. While this is being done, the stomach keeps working the food much as a baker kneads dough. This is done to mix the gastric juice with the food.
~6.~ After an hour or two the stomach squeezes the food so hard that a little of it, which has been digested by the gastric juice and the saliva, escapes through the lower opening, the pylorus, of which we have already learned. As the action of the stomach continues, more of the digested food escapes, until all that has been properly acted upon has pa.s.sed out.
~7. Intestinal Digestion.~--We sometimes eat b.u.t.ter with bread, or take some other form of fat in our food. This is not acted upon by the saliva or the gastric juice. When food pa.s.ses out of the stomach into the small intestine, a large quant.i.ty of bile is at once poured upon it. This bile has been made beforehand by the liver and stored up in the gall-bladder.
The bile helps to digest fats, which the saliva and the gastric juice cannot digest.
~8.~ The pancreatic juice does the same kind of work that is done by the saliva, the gastric juice, and the bile. It also finishes up the work done by these fluids. It is one of the most important of all the digestive juices.
~9.~ The intestinal juice digests nearly all the different elements of the food, so that it is well fitted to complete the wonderful process by which the food is made ready to enter the blood and to nourish the body.
~10.~ While the food is being acted upon by the bile, the pancreatic juice, and intestinal juice, it is gradually moved along the intestines.
After all those portions of food which can be digested have been softened and dissolved, they are ready to be taken into the blood and distributed through the body.
~11. Absorption.~--If you put a dry sponge into water, it very soon becomes wet by soaking up the water. Indeed, if you only touch a corner of the sponge to the water, the whole sponge will soon become wet. We say that the sponge absorbs the water. It is in a somewhat similar way that the food is taken up or absorbed by the walls of the stomach and intestines. When the food is absorbed, the greater part of it is taken into the blood-vessels, of which we shall learn in a future lesson.
~12. Liver Digestion.~--After the food has been absorbed, the most of it is carried to the liver, where the process of digestion is completed.
The liver also acts like an inspector to examine the digested food and remove hurtful substances which may be taken with it, such as alcohol, mustard, pepper, and other irritating things.
~13. The Thoracic Duct.~--A portion of the food, especially the digested fats, is absorbed by a portion of the lymphatic vessels called _lacteals_, which empty into a small vessel called the _thoracic duct_.
This duct pa.s.ses upward in front of the spine and empties into a vein near the heart.
SUMMARY.
How a mouthful of food is digested:
1. It is first masticated--that is, it is chewed and moistened with saliva.