* * * * * (5) Rennin and Renninogen.--Rennin is the milk-curdling ferment of the gastric juice. It is derived from renninogen through the action of hydrochloric acid. Lime salts also possess the power of transforming renninogen into the active ferment. Deficiency of rennin has the same significance as deficiency of pepsin, and is more easily recognized. Since the two enzyms are almost invariably present or absent together, the test for rennin serves also as a test for pepsin. * * * * * Test for Rennin.--Neutralize 5 c.c. filtered gastric juice with very dilute sodium hydroxid solution; add 5 c.c. fresh milk, and place in an incubator or in a vessel of water at about 104° F. Coagulation of the milk in ten to fifteen minutes shows a normal amount of rennin. Delayed coagulation denotes a less amount. {223} Test for Renninogen.--To 5 c.c. neutralized gastric juice add 2 c.c. of 1 per cent. calcium chlorid solution and 5 c.c. fresh milk, and place in an incubator. If coagulation occurs, renninogen is present. * * * * * (6) Blood.--Blood is present in the vomitus in a great variety of conditions. When found in the fluid removed after a test-meal, it commonly points toward ulcer or carcinoma. Blood can be detected in nearly one-half of the cases of gastric cancer. The presence of swallowed blood must be excluded. * * * * * Test for Blood in Stomach-contents.--To 10 c.c. of the fluid add a few cubic centimeters of glacial acetic acid and shake the mixture thoroughly with an equal volume of ether. Separate the ether and apply to it the guaiac test (p. 89); or evaporate and apply the hemin test (p. 202) to the residue. When brown particles are present in the fluid, the hemin test should be applied directly to them. * * * * * 2. Quantitative Tests.--(1) Total Acidity.--The acid-reacting substances which contribute to the total acidity are free hydrochloric acid, combined hydrochloric acid, acid salts, mostly phosphates, and, in some pathologic conditions, the organic acids. The total acidity is normally about 50 to 75 _degrees_ (see method below), or, when estimated as hydrochloric acid, about 0.2 to 0.3 _per cent._ * * * * *