780 PHYSIOLOGY OF DIGESTION AND SECRETION. 



periments it is customary to employ a glycerin or commercial prep- 

 aration of the gastric mucous membrane, and to add a small portion 

 of this preparation to a large bulk of 0.2 per cent, hydrochloric acid. 

 The artificial juice thus made, when kept at a temperature of from 

 37 to 40 C., will digest proteins rapidly if the preparation of pepsin 

 is a good one. While the strength of the acid employed is generally 

 from 0.2 to 0.3 per cent., digestion will take place in solutions of 

 greater or less acidity. Too great or too small an acidity, however, 

 will retard the process; that is, there is for the action of the pepsin 

 an optimum acidity which, according to the experiments of Mich- 

 aelis,* is represented by a concentration of hydrogen ions of about 

 4 X 10- 2 (0.04 n). 



The Pepsin-hydrochloric Acid Digestion of Proteins. It has 

 long been known that solid proteins, when exposed to the action of a 

 normal or an artificial gastric juice, swell up and eventually pass 

 into solution. The soluble protein thus formed was known not to be 

 coagulated by heat, and was remarkable also for being more diffus- 

 ible than other forms of soluble proteins. This end-product of diges- 

 tion was formerly conceived as a soluble protein with properties 

 fitting it for rapid absorption, and the name of peptone was given to 

 it. It was quickly found, however, that the process is complicated 

 that in the conversion to so-called "peptone" the protein under 

 digestion passes through a number of intermediate stages. The in- 

 termediate products were partially isolated and were given specific 

 names, such as acid-albumin, parapeptone, and propeptone. The 

 present conception of the process we owe chiefly to Kiihne. This 

 author believed that the protein passes through three general stages 

 before reaching the final condition of peptone. This view is indi- 

 cated briefly by the following schema : 



Native protein. 



Acid albumin (syntonin) . 



Primary proteoses (protalbumoses). 



Secondary proteoses (deutero-albumoses). 



Peptone. 



The first step is the conversion of the protein to an acid albumin. 

 This change may be considered as being chiefly an effect of the hy- 

 drochloric acid, although in some way the combined action of the 

 pepsin-hydrochloric acid compound is more effective than a solution 

 of the acid alone of the same strength. Like the acid albumins 

 (metaproteins) in general (see Appendix), the syntonin is readily 

 precipitated on neutralization. In the beginning of peptic diges- 

 tion, therefore, if the solution is neutralized with dilute alkali, 

 an abundant precipitate of syntonin occurs. Later on in the 

 digestion, neutralization gives no such effect the syntonin 



* Michaelis and Mendelssohn, "Biochemische Zeitschrift," 65, 1, 1914. 



