360 HANDBOOK OF PHYSIOLOGY. 



action of hydrochloric acid, even in minute traces, whereas lactic and 

 other organic acids have no such action. Pepsin will act with phos- 

 phoric, lactic, and oxalic acids, as proven by laboratory experiments, but 

 the best results are obtained with hydrochloric acid. Of these tests the 

 following may be mentioned. 



An aqueous alkaline solution of 00 tropceolin, a bright yellow dye, is 

 turned red on the addition of a minute trace of hydrochloric acid ; and 

 aqueous solutions of methyl violet and gentian violet are turned blue un- 

 der the same circumstances. The lactic acid sometimes present in the 

 contents of the stomach is derived partly from the sarcolactic acid of 

 muscle, and partly from lactic acid fermentation of carbohydrates. Lactic 

 acid (C 3 H 6 3 ), if present, gives the following test. A solution of 10 cc. 

 of a 4 per cent aqueous solution of carbolic acid, 20 cc. of water, and one 

 drop of liquor ferri perchloridi is made, forming a blue-colored mixture ; 

 a mere trace of free lactic acid added to such a solution causes it to 

 become yellow, whereas hydrochloric acid even in large amount only 

 bleaches it. 



The proteid matter in the food combines with part of the hydro- 

 chloric acid, which is then known as combined acid and does not redden 

 litmus paper. As this combination is immediate, it follows that no free 

 acid is found in the gastric contents until the amount secreted is more 

 than enough to saturate the various albuminous affinities. It is for this 

 reason that, as already mentioned, salivary digestion may continue in 

 the stomach for some time after the commencement of gastric digestion. 

 According to Ehiiich the amount necessary to saturate the affinities of 

 100 grammes of various articles of diet is as follows: 



Beef (boiled) . . . . . 2.0 grammes of pure HC1. 



Mutton (boiled) 1.9 " 



Veal (boiled) 2.2 " 



Pork (boiled) 1.6 " 



Ham (boiled) 1.8 " 



Sweatbread (boiled) . . . .0.9 



Wheat bread 0.3 



Eye bread 0.5 " 



Swiss cheese . . . . . 26 " 

 Milk (100 cc.) . . . . 0.32-0.42 " 



As regards the formation of pepsin and acid, the former is produced 

 by the central or chief cells of the cardiac glands, and also most likely 

 by the similar cells in the pyloric glands ; the acid is chiefly found at the 

 surface of the mucous membrane, but is in all probability formed by the 

 parietal cells of the cardiac glands, hence called oxyntic, as no acid is 

 formed by the pyloric glands in which this variety of cell is absent. 



The acid is probably formed from materials in the blood and results 

 from a combination of common salt with monosodic orthophosphate 



