Starch.—(a) Place a small lump of starch in one fourth of a pint of water and heat gradually to boiling, stirring well. Then add enough water to form a thin liquid and fill a test tube half full. Add to this a few drops of a solution of iodine. (Prepare by dissolving a crystal of iodine in 25 cubic centimeters (1/20 pint) of a solution of potassium iodide in water and add water to this until it is a light amber color.) The starch solution is turned blue, (b) Cut with a razor a thin slice from a potato. Place this in a weak solution of iodine for a few minutes and then examine with the microscope, using first a low and then a high power. Numerous starch grains inclosed in cellulose walls will be seen (Fig. 60).
Dextrose, or Grape Sugar.—Place a solution of the substance supposed to contain grape sugar in a test tube and add a few drops of a dilute solution of copper sulphate. Then add sodium hydroxide solution until the precipitate which first forms is redissolved and a clear blue liquid obtained. Heat the upper portion of the liquid slowly to near the boiling point. A little below the boiling point the blue color disappears and a yellow-red precipitate is formed. If the upper layer of[pg 137] the liquid is now boiled, the color deepens and this may be contrasted with the blue color below. Apply this test to the sugar in raisins and in honey.
Fat.—Fat is recognized by its effect on paper, making a greasy stain which does not disappear on heating and which renders the paper translucent. Try butter, lard, or olive oil. Also show the presence of fat in peanuts by crushing them in a mortar and rubbing the powder on thin paper. If the substance to be tested contains but little fat, this may be dissolved out with ether. If a drop of ether containing the fat is placed on paper, it evaporates, leaving the fat, which then forms the stain.
To show the Effect of Alcohol upon Proteid.—Place some of the white of a raw egg in a glass vessel and cover it with a small amount of alcohol. As the albumin (proteid) hardens, or coagulates, observe that the quantity of clear liquid increases. This is due to the withdrawal of water from the albumin by the alcohol. Since the tissues are made up chiefly of proteids, a piece of muscle or of liver may be used in the experiment, instead of the egg, with similar results.
To illustrate the Digestive Process.—To a tumbler two thirds full of water add a little salt. Stir and observe that the salt is dissolved. Taste the solution to see that the salt has not been changed chemically. Now add a little powdered limestone to the water and stir as before. Observe that the limestone does not dissolve. Then add some hydrochloric acid and observe the result. State the part played by the acid and by the water in dissolving the limestone. Apply to the digestion of the different classes of foods.
CHAPTER X - ORGANS AND PROCESSES OF DIGESTION
The organs of digestion are adapted to the work of dissolving the foods by both their structure and arrangement. Most of them consist either of tubes or cavities and these are so connected, one with the other, as to form a continuous passageway entirely through the body. This passageway is known as
The Alimentary Canal. —The alimentary canal has a length of about thirty feet and, while it begins at the mouth, all but about eighteen inches of it is found in the abdominal cavity. On account of its length it lies for the most part in coils, the two largest ones being known as the small intestine and the large intestine. Connected with the alimentary canal are the glands that supply the liquids for acting on the food. The divisions of the canal and most of the glands that empty liquids into it are shown in Fig. 63 and named in the table below: