(over 99 per cent), free hydrochloric acid and a digestive fer ment called pepsin. 128. Muscles of the stomach. The chief function of the human stomach is to secrete the gastric juice and to mix this juice thoroughly with the food. The muscular walls are well adapted for this purpose. When the food reaches the stomach, the gastric juice oozes out upon it, and the mixture is pushed back and forth and up and down by the successive action of the different layers of muscles. The return of the food to the mouth cavity is prevented by the contraction of the circular muscles at the lower end of the gullet, except in the case of nausea, when they relax and allow the stomach to rid itself of its contents. The circular muscle at the pyloric end of the stomach (Fig. 26) relaxes from time to time, and the partially digested food is pushed on into the intestine. Fortunately for the well-being of the body, all these processes are entirely automatic; that is, they are carried on without our conscious direction. The muscles of the alimentary canal for this reason are called involuntary (Latin, in without + voluntas = will). = 129. Digestion in the stomach. The gastric juice has practically no effect on the nutrients starch and fat. The saliva, however, that is mixed with the food and swallowed with it continues to act upon the starch for a time, particularly in the upper part of the stomach. Sugars and soluble salts (that is, salts that dissolve in water), if not dissolved in the mouth, are readily liquefied by the water of the gastric juice. Certain mineral food substances, however, like phosphate of lime found in milk, are not soluble in water, and these insoluble salts reach the stomach unchanged. The following experiment illustrates the way in which mineral matters are made liquid by the hydrochloric acid in the gastric juice. - 130. Digestion of mineral matters. (Optional.) Laboratory demonstration. Note to Teacher: Part A should be demonstrated in connection with the study of saliva; Part B, in connection with gastric digestion. Materials: Table salt, phosphate of lime, diluted hydrochloric acid (one part acid to six parts water). A. Soluble mineral matters. 1. Put some table salt into a test tube, add water, and shake well. Does the salt dissolve? How do you know? 2. Saliva is largely (over 99 per cent) composed of water. How, then, are soluble mineral matters made liquid in the mouth? B. Insoluble mineral matters. 1. Put some insoluble mineral matter like phosphate of lime (which is one of the constituents of milk) into a test tube, add water, and shake well, then allow the tube to stand for a time before answering the following questions. a. Does phosphate of lime dissolve in water? How do you know? Why is phosphate of lime called an insoluble mineral matter? b. Shake the mixture again and add some diluted hydrochloric acid. What change do you observe? 2. Hydrochloric acid is one of the ingredients of gastric juice. How, then, are insoluble mineral matters like phosphate of lime digested in the stomach? 131. Digestion of proteins. One of the most important actions which takes place in the stomach is the digestion of proteins. This class of nutrients is not readily soluble in water and so cannot pass through the walls of cells (P. B., 52). Hence, before proteins can be made available for use in the body they must be changed to a soluble form known as peptone (P. B., 5). This chemical change is brought about in our bodies to some extent by the gastric juice. 132. Digestion of proteins. Optional laboratory demonstration. Materials: Boiled egg, powdered pepsin (which should be obtained fresh or kept in a tightly stoppered bottle), hydrochloric acid, water; test tubes. Each of the following experiments should be kept throughout the whole time as nearly as possible at the temperature of the body (98.6° F.). A. To prove that protein requires digestion after it is swallowed. 1. Shave off with a knife and cut into the finest pieces possible a part of the white of a boiled egg (or better, grate the egg). The solid constituents of egg are largely protein. Put into a test tube a small amount (about twice the size of a pea) of this minced egg, add water, and shake. Label the test tube No. 1, and allow the mixture to stand for a day or two as nearly as possible at a temperature of 98.6° F. (which is the normal temperature of the interior of our bodies). a. Has all the egg been made liquid or digested by the water? How do you know ? b. Pour off some of the clear liquid into a test tube, and add nitric acid and boil. Has any of the protein been digested? How do you know ? 2. Into another test tube put the same amount of minced egg, add a spoonful or more of saliva. Label it test tube No. 2. Shake and allow it to stand for a day or two beside test tube No. 1. a. Is protein digested by saliva? How do you know? B. To prove that gastric juice digests protein. 1. Into a third test tube put a small amount of the minced egg. Half fill the tube with water, add powdered pepsin to the amount equal to about the size of a pea, and also add five to ten drops of diluted hydrochloric acid. (Water, pepsin, and hydrochloric acid are the three principal ingredients of gastric juice.) Label the test tube No. 3, shake the mixture, and put it in a warm place beside test tubes 1 and 2. (Since it is difficult to get the exact proportion of the three ingredients of gastric juice, it is well to prepare several tubes as described above, labelling each test tube No. 3.) At the end of a few hours or a day examine the test tubes containing the minced egg and the artificial gastric juice, comparing them with test tubes 1 and 2. Has the egg been digested? How do you know? V. THE SMALL INTESTINE AND ITS FUNCTIONS 133. Position, form, and size. The small intestine is a much-coiled tube, filling the larger portion of the abdominal cavity (Fig. 2). It is usually twenty feet or more in length, and therefore constitutes nearly four fifths of the whole length of the alimentary canal. Beginning at the stomach, it decreases somewhat in size until it opens into the large intestine. 134. Peritoneum. The whole abdominal cavity is lined with thin, smooth membrane called the peritoneum. Sheets of peritoneum likewise inclose the various organs found in the abdominal cavity, and help to connect these organs to the walls of the abdomen. Peritonitis is an inflammation of any portion of this membrane. 135. Digestion in the small intestines. In the intestines. important digestive processes are carried on (1) by the juices secreted in the glands found in the inner wall of the intestine (intestinal glands), (2) by the pancreatic juice secreted by the pancreas, and (3) by the bile secreted by the liver. All these juices, when mixed with the food in the intestine, H bring about the digestion of fats and complete the digestion of starch and proteins. The pancreas (Fig. 26) lies just below the stomach and extends from the region of the pylorus toward the left side of the body. Within the gland is secreted the pancreatic juice, which is poured out through a duct upon the food just after it enters the small intestine. Pancreatic juice digests three of the nutrients; namely, starch, proteins, and fats. Like saliva, pancreatic juice changes starch into sugar, and like gastric juice, it converts proteins into peptones. The heat of the body melts much of the fat before it reaches the intestine, but this liquid cannot be absorbed until it has been still further acted upon chemically by the pancreatic juice and bile. VI. THE LARGE INTESTINE AND ITS FUNCTIONS 136. Position, form, and size. The large intestine is the last portion of the alimentary canal. It is a tube five or six feet long, with a gradually decreasing diameter. Beginning in the lower righthand region of the abdominal cavity as a sac-like pouch (Fig. 26), the large intestine passes upward on the right side of the body cavity to the lower surface of the stomach; it then crosses the abdominal cavity; a third portion continues downward on the left side. The large intestine then takes an S-shaped course and passes to the exterior of the body by a short, straight tube. 137. Vermiform appendix. - On the right side of the body, and connected with the beginning of the large intestine, is a small, tubular sac about the size of a lead pencil, and usually about four inches long (Fig. 26). From its more or less twisted shape it has received the name vermiform appendix (Latin, vermiform = worm-shaped). Appendicitis is a diseased condition arising from inflammation in the tissues of the appendix. VII. ABSORPTION FROM THE ALIMENTARY CANAL 138. Necessity for the absorption of food. We have now learned something of the processes of digestion. We |