It is well known that “rennet” prepared from the calf’s stomach has a remarkable effect in rapidly curdling milk, and this property is utilized in the manufacture of cheese.  Now, a similar ferment is abundant in the gastric juice, and may be called rennin.  It causes milk to clot, and does this by so acting on the casein as to make the milk set into a jelly.  Mothers are sometimes frightened when their children, seemingly in perfect health, vomit masses of curdled milk.  This curdling of the milk is, however, a normal process, and the only noteworthy thing is its rejection, usually due to overfeeding.

Experiment 58. To show that pepsin and acid are necessary for gastric digestion. Take three beakers, or large test tubes; label them A, B, C.  Put into A water and a few grains of powdered pepsin.  Fill B two-thirds full of dilute hydrochloric acid (one teaspoonful to a pint), and fill C two-thirds full of hydrochloric acid and a few grains of pepsin.  Put into each a small quantity of well-washed fibrin, and place them all in a water bath at 104 degrees Fahrenheit for half an hour.

Examine them.  In A, the fibrin is unchanged; in B, the fibrin is clear and swollen up; in C, it has disappeared, having first become swollen and clear, and completely dissolved, being finally converted into peptones.  Therefore, both acid and ferment are required for gastric digestion.

Experiment 59.  Half fill with dilute hydrochloric acid three large test tubes, labelled A, B, C.  Add to each a few grains of pepsin.  Boil B, and make C faintly alkaline with sodic carbonate.  The alkalinity may be noted by adding previously some neutral litmus solution.  Add to each an equal amount—­a few threads—­of well-washed fibrin which has been previously steeped for some time in dilute hydrochloric acid, so that it is swollen and transparent.  Keep the tubes in a water-bath at about 104 degrees Fahrenheit for an hour and examine them at intervals of twenty minutes.

  After five to ten minutes the fibrin in A is dissolved and the fluid
  begins to be turbid.  In B and C there is no change.  Even after long
  exposure to 100 degrees Fahrenheit there is no change in B and C.

After a variable time, from one to four hours, the contents of the stomach, which are now called chyme, begin to move on in successive portions into the next part of the intestinal canal.  The ring-like muscles of the pylorus relax at intervals to allow the muscles of the stomach to force the partly digested mass into the small intestines.  This action is frequently repeated, until even the indigestible masses which the gastric juice cannot break down are crowded out of the stomach into the intestines.  From three to four hours after a meal the stomach is again quite emptied.

A certain amount of this semi-liquid mass, especially the peptones, with any saccharine fluids, resulting from the partial conversion of starch or otherwise, is at once absorbed, making its way through the delicate vessels of the stomach into the blood current, which is flowing through the gastric veins to the portal vein of the liver.