DIGESTION IN THE STOMACH 389 



whether hydrochloric or some abnormal organic acid, such as lactic, is 

 present; the total acidity; whether the acid is free or combined; and 

 how much in each state. Many methods have been devised for this 

 purpose. Opinion varies as to whether it is important for the hydro- 

 chloric acid to be free. Apparently it matters little, from the digestive 

 point of view, whether it is free or combined; but in the combined 

 state the protective power of the free acid against bacteria is lacking. 

 The amount of free to combined acid will depend upon 



1. The amount of secretion. 



2. The amount of protein in the stomach. 



3. The rate of empty ing. 



4. The rate of absorption. 



5. The amount of mixing of the contents. 



For testing the total acidity, the best indicators are probably 

 phenolphthalein (which gives a slightly high reading) and rosolic acid. 

 The amount of free acid is obtained by the use of such indicators as 

 congo-red (turned blue by free acid), Giinzburg's reagent, dimethyl- 

 aminoazobenzol (Topfer's reagent), tropseolin OO. The amount of 

 the combined acid is obtained by deducting the free from the total 

 acidity. An excellent method is that known as the method of deficit 

 and excess. 



Method of Excess. Ten c.c. of the contents are taken. If free acid 

 is present, this is estimated by running in ^ NaOH until neutralized, 

 and testing from time to time by removing a small drop to congo-red 

 paper, which is no longer turned blue when the neutralization point is 

 reached. A few drops of phenolphthalein are then added, and the 

 acidity to this indicator measured. This amount gives the combined 

 acidity, that with congo-red the free acidity, and the two together 

 give the total acidity. 



The Method of Deficit. If by testing with congo-red it is found tha* 

 free acid is absent, 10 c.c. of the contents are taken, and -^ HC1 added 

 until free acid makes its appearance to congo-red. The amount 

 added is noted, and this gives the deficit. The sample is then 

 exactly neutralized by a corresponding amount of T ^ NaOH, phenol- 

 phthalein added, and the acidity to this indicator measured, this giving 

 the amount of the combined and also the total acidity. 



Physiologically active HC1 may also be determined (A) by mixing 

 the filtered gastric contents with excess of sodium bicarbonate, drying, 

 incinerating, and determining the total chlorides in the ash by 

 Volhard's method (see Urine, p. 462). The process is now repeated 

 without the addition of sodium bicarbonate (B). In this case only 

 the mineral chlorides are retained in the ash. A - B gives the physio- 

 logically active HC1. 



The amount of enzyme may be estimated roughly by determining 

 the action of the rennin present. The rennin is assumed to run parallel 

 with the pepsin. In several tubes 5 c.c. of milk and 2-5 c.c. of 1 per 

 cent, calcium chloride are taken. To these are added 5 c.c of the gastric 



