Experiment 68. To show the action of gastric juice on milk. Mix two teaspoonfuls of fresh milk in a test tube with a few drops of neutral artificial gastric juice;[[30]] keep at about 100° F. In a short time the milk curdles, so that the tube can be inverted without the curd falling out. By and by whey is squeezed out of the clot. The curdling of milk by the rennet ferment present in the gastric juice, is quite different from that produced by the “souring of milk,” or by the precipitation of caseinogen by acids. Here the casein (carrying with it most of the fats) is precipitated in a neutral fluid.
Experiment 69. To the test tube in the preceding experiment, add two teaspoonfuls of dilute hydrochloric acid, and keep at 100° F. for two hours. The pepsin in the presence of the acid digests the casein, gradually dissolving it, forming a straw-colored fluid containing peptones. The peptonized milk has a peculiar odor and bitter taste.
Experiment 70. To show the action of rennet on milk. Place milk in a test tube, add a drop or two of commercial rennet, and place the tube in a water-bath at about 100° F. The milk becomes solid in a few minutes, forming a curd, and by and by the curd of casein contracts, and presses out a fluid,—the whey.
Experiment 71. Repeat the experiment, but previously boil the rennet. No such result is obtained as in the preceding experiment, because the rennet ferment is destroyed by heat.
Experiment 72. To show the effect of the pancreatic ferment (trypsin) upon albuminous matter. Half fill three test tubes, A, B, C, with one-per-cent solution of sodium carbonate, and add 5 drops of liquor pancreaticus, or a few grains of Fairchild’s extract of pancreas, in each. Boil B, and make C acid with dilute hydrochloric acid. Place in each tube an equal amount of well-washed fibrin, plug the tubes with absorbent cotton, and place all in a water-bath at about 100° F.
Experiment 73. Examine from time to time the three test tubes in the preceding experiment. At the end of one, two, or three hours, there is no change in B and C, while in A the fibrin is gradually being eroded, and finally disappears; but it does not swell up, and the solution at the same time becomes slightly turbid. After three hours, still no change is observable in B and C.
Experiment 74. Filter A, and carefully neutralize the filtrate with very dilute hydrochloric or acetic acid, equal to a precipitate of alkali-albumen. Filter off the precipitate, and on testing the filtrate, peptones are found. The intermediate bodies, the albumoses, are not nearly so readily obtained from pancreatic as from gastric digests.
Experiment 75. Filter B and C, and carefully neutralize the filtrates. They give no precipitate. No peptones are found.
Experiment 76. To show the action of pancreatic juice upon the albuminous ingredients (casein) of milk. Into a four-ounce bottle put two tablespoonfuls of cold water; add one grain of Fairchild’s extract of pancreas, and as much baking soda as can be taken up on the point of a penknife. Shake well, and add four tablespoonfuls of cold, fresh milk. Shake again.
Now set the bottle into a basin of hot water (as hot as one can bear the hand in), and let it stand for about forty-five minutes. While the milk is digesting, take a small quantity of milk in a goblet, and stir in ten drops or more of vinegar. A thick curd of casein will be seen.
Upon applying the same test to the digested milk, no curd will be made. This is because the pancreatic ferment (trypsin) has digested the casein into “peptone,” which does not curdle. This digested milk is therefore called “peptonized milk.”
Experiment 77. To show the action of bile. Obtain from the butcher some ox bile. Note its bitter taste, peculiar odor, and greenish color. It is alkaline or neutral to litmus paper. Pour it from one vessel to another, and note that strings of mucin (from the lining membrane of the gall bladder) connect one vessel with the other. It is best to precipitate the mucin by acetic acid before making experiments; and to dilute the clear liquid with a little distilled water.