Experiment 117. -Chop a piece of the white of a hard-boiled egg into pieces about as large as the head of a pin and place in a test tube. Chop up another piece much finer than this and place it in a second test tube. Make a mixture of 100 cc. of water, 5 cc. of essence of pepsin and 2 cc. of hydrochloric acid. Pour into each test tube enough of this mixture to cover the white of egg to a considerable depth. Shake thoroughly and put in a place where the temperature can be maintained at 37° C. or 98° F. A fireless cooker or a bucket of warm water is good for this. Allow to stand for several hours, keeping the temperature constant. The white of egg is dissolved, the action being more rapid in the second tube. Try the same experiment using water; using dilute hydrochloric acid. Do these have the same effect as when used with the pepsin? The pepsin solution is an artificial gastric juice.
In order that the work of the body may be carried on, food is required. This food may be supplied by either animals or plants. The original source of all animal and plant food, as has been seen, is in the chlorophyll manufactory of the leaf and green stem. After this leaf food has been manufactured, it is simply modified by the plants and animals through which it passes. The food is used (1) in growing new cells, (2) in repairing cells that have been used up or destroyed, (3) in providing energy to carry on the activities of the body and maintain its heat or(4) in doing external work, such as moving the body itself from place to place or moving other bodies.
To furnish any of this energy, the cells must be able to combine food with oxygen. To do this the food must be digested or prepared so that it can pass through animal tissue. In the higher animals, a complicated apparatus is provided to accomplish this. In man, it is briefly as follows: a long continuous tube, the food-tract or the alimentary canal (Fig. 109), extends through the body. Different portions of this tube are adapted to different processes. In the mouth, the teeth grind the food into small bits and mix it with the saliva.
Fig. 109.
This is an exceedingly important part of the process, because if thefood is not ground fine, the digestive juices cannot readily get at it, and the whole process of digestion is greatly retarded. Thus much more energy is expended than otherwise would be. The saliva is necessary to digest some of the starch and to aid in the further digestion.
The food passes from the mouth down the throat and through a valve into the stomach. This is a large pouch which will hold usually from three to four pints. It has muscular walls which enable it to contract and expand, thus keeping the food moving about so that it is thoroughly mixed with the gastric juice. The gastric juice is secreted by little glands thickly imbedded in the lining of the stomach. Artificial gastric juice was made in Experiment 117. Some of the proteins (Experiment 119)are digested in the stomach, although the larger part of digestion takes place in the small intestine.
From the stomach the food passes through a valve into the smallintestine. This is a complexly coiled tube which fills the larger part of the abdomen. The inner wall of the tube is lined with glands which secrete digestive juices, and into the intestine are poured the secretions from two large glands, the pancreas and the liver. The smallintestine is the great digestive organ of the body. Here the fats and oils (Experiment 120) are digested and the digestion of the starches and proteins is completed. The small intestine opens through a valve into the large intestine, a tube five or six feet long decreasing in size toward the exit to the body. There is little digestion in the large intestine.
The changes that take place in the food as it passes through the alimentary canal are very complex, but during its progress the valuable part of the food is so changed and prepared that it can be absorbed by the blood and transported by it to the different parts of the body where its energy is needed. Absorption takes place all along the alimentary canal wherever the food has been sufficiently prepared.
110.Necessary Foods.
Experiment 118. -Place in different test tubes small amounts of (1) corn starch, (2) grape sugar, (3) scrapings from a raw potato, (4) flour, and (5) the white of an egg. Pour in a little water and shake thoroughly. Drop into each tube a few drops of the iodine solution prepared in Experiment 100.
Experiment 119. -Place in test tubes small quantities of (1) the white of a hard-boiled egg, (2) tallow or lard, (3) grape sugar, and (4) any other food which may be handy. Pour a little concentrated nitric acid into each tube and allow to stand for a minute. Be careful not to get the nitric acid on the clothes or hands. Pour the acid out into a slop jar and wash the substances with a little water. Pour off the wash water and pour on a little strong ammonia. If the substances turn a yellow or orange color, proteins are present. Which substances contain proteins?
Experiment 120. -Gasoline vapor is very inflammable, so be sure in this experiment that there is no flame in the room. Place about a spoonful of (1) both the white and the yellow of an egg, (2) flaxseed meal, (3) yellow corn meal, (4) white flour, and (5) other foods it is desired to test in separate evaporating dishes or beakers near an open window. Pour on to these enough gasoline to more than cover them and stir thoroughly. Cover the evaporating dishes and allow to stand for ten or fifteen minutes. Pour the gasoline offinto a beaker and set the beaker outside the window until the gasoline has evaporated. If there is anything left it must have been dissolved from the food. If a substance remains, place a drop of it on a piece of paper. Smell of it. Try to mix it with water. Rub it between the fingers. Try any other fat or oil test of which you can think.