DIGESTION IN THE STOMACH 385 



Degree of Degree of Duration of Secretion 



Acidity. Digestive Activity. of Juice. 



Meat . . . . Bread . . . . Bread. 



Milk ... .. Meat .. .. Meat 



Bread . . . . Milk . . . . Milk 



The Digestive Processes in the Stomach Peptic Digestion. The 

 peptic enzyme is proteolytic in nature, and acts only in acid medium 

 {HC1 0-1 to 0-5 per cent.); in fact, it is probable that the true digestive 

 principle is a combination pepsin-hydrochloric acid. The protein is 

 first swollen and partially disintegrated by the hydrochloric acid, and 

 then acted upon by the pepsin-hydrochloric acid. This can be shown 

 by the folio whig experiments: (1) Coagulated egg-albumin soaked in 

 0-3 HC1, and then washed and added to neutral pepsin, is not digested; 

 {2) if the swelling of protein be stopped by the presence of bile, diges- 

 tion is stopped or greatly hindered. 



The peptic ezyme digests all proteins except keratin. Elastin is 

 but little attacked. The other proteins, including gelatin, are easily 

 digested, passing through the preliminary stages qf conversion into 

 proteoses, primary and secondary, to peptones and peptides (abiuretic 

 bodies not giving the biuret test). The change is by no means so 

 complete as in the case of the pancreatic enzyme, trypsin. It may be 

 represented as follows : 



Protein - > -- Acid metaprotein 



Primary proteoses (proto and hetero) 



Secondary proteoses (A, B, C) 



sU 

 Peptones (A, B) 



4- 

 Peptides (giving no biuret test) . 



The methods by which the presence of these bodies can be shown in 

 the digest can be gathered from the table on p. 386, which embodies 

 the chief facts ascertained by recent research. It will be seen that 

 the chief differences between them are brought out by their relative 

 solubility on addition of a saturated solution of ammonium sulphate, 

 salicylsulphonic acid, or alcohol. 



It has been found that only a small percentage of protein is 

 converted into acid metaprotein (formerly called " syntonin "). 

 Some of the primary proteoses are split off earlier than the others; 

 the same remark applies to the secondary proteoses and peptones. 

 Therefore, quite early in the digestive process, peptones and peptides 

 make their appearance. This is to be explained by the fact that these 

 bodies are not of the same chemical composition. For example, hetero- 

 proteose yields, on decomposition, chiefly leucin and glycin, whilst 

 proto proteose yields much tyrosin and indol. Similarly, with the 

 secondary proteoses, one was found to contain much sulphur, another 



