118 PHYSIOLOGY OF ALIMENTATION. 



in fibrin alone. As is well known, fibrin swells in different 

 acids, but not to the same degree in each. If now the 

 acids are arranged according to the degree in which they 

 make fibrin take up water (swell), this order is the same 

 as that given above for the effect of these same acids on 

 digestion under the influence of acid-proteinase. Sulphuric 

 acid, for example, which stands very low in the list of acids 

 as favoring acid-proteinase digestion, occupies a similar 

 position when the acids are arranged in the order in which 

 they cause fibrin to swell. What has been said of the acids 

 holds also for the salts. The more a salt inhibits the absorp- 

 tion of water by fibrin (for example, a sulphate) the more 

 does this salt retard the digestion of a protein. 



The effect of various external conditions upon the proteo- 

 lytic activity of acid-proteinase has usually been attributed 

 to the effect of these conditions upon the acid-proteinase 

 itself. Unquestionably external conditions can most markedly 

 influence the state of the ferment itself it is no doubt a 

 colloid and influenced, as are all colloids, by external con- 

 ditions but in the experiments which have been cited, the 

 chief effect of the external conditions seems to have been 

 on the protein undergoing digestion. BRUCKE many years 

 ago pointed out that the more fibrin has swelled the more 

 rapidly it is digested. The different acids and salts in- 

 fluence this swelling in different degrees, in consequence of 

 which the protein is attacked by the acid-proteinase with 

 greater or less ease. It seems to me that this difference 

 between the physical change which a protein suffers when 

 it swells during the process of peptic digestion, and the 

 chemical change which probably constitutes the real activity 

 of the proteolytic ferment when the simpler digestion- 

 products are formed, has never been sufficiently well drawn. 

 The two are different, and differently influenced by external 

 conditions. 



The optimum concentration of hydrochloric acid for the 

 activity of acid-proteinase varies with the protein which 



