THE DIGESTION OF THE FOODSTUFFS 131 



5. Upon the presence of the products of digestion. All 

 fermentation processes are hindered and finally arrested by 

 the accumulation of the resulting products. In peptic diges- 

 tion, the amount of accumulated proteoses must be very 

 large before it completely stops digestion. This influence 

 of the products of digestion is not felt in the stomach because 

 the products formed are rapidly removed. 



The products of digestion can also hinder the digestion 

 by uniting with the hydrochloric acid and thereby rendering 

 it inactive. In this case, free hydrochloric acid is lacking 

 and the addition of acid starts the digestion again. 



6. Salts can inhibit peptic digestion either by preventing 

 the inbibition or by precipitating the pepsin. This last is 

 also accomplished by alcohol in strong concentration. 



Auto-digestion of the stomach. A piece of the mucosa of the 

 stomach heated in 0.2% hydrochloric acid at 40 C. digests itself. 

 Why normally the mucous lining of the stomach does not digest 

 itself has received no satisfactory answer; it probably depends 

 upon the specific vital character of the epithelial cell of the 

 mucosa. 



(b) The inversion of cane-sugar. Cane-sugar is inverted 

 in the stomach, i.e. it is split up by the free hydrochloric 

 acid into dextrose and levulose. 



(c] The coagulation of caseinogen. The caseinogen of the 

 milk coagulates in the stomach, previous to its digestion. 

 This coagulation is brought about by the rennin, which splits 

 the caseinogen up into casein and a soluble proteid called 

 whey proteid. Casein unites with calcium, forming an in- 

 soluble compound, cheese. Hence calcium is necessary for 

 the coagulation of caseinogen, and coagulation can be pre- 

 vented by precipitation of the calcium salts, e.g. by oxalates. 



Rennin, a ferment of unknown chemical composition, can 

 act in an acid, alkaline, or neutral medium. 



Rennet, used in the manufacture of cheese, can be extracted 

 from the stomachs of calves. 



As to the purpose of milk coagulation, see page 14.3. 

 Besides the above-named digestive actions, the gastric 

 juice has the special function of disinfectant. Pathogenic and 



