PEPSIN. 



333 



was formerly credited with a powerful checking action, but, as shown 

 by Kiihne, 1 while powerful in preventing the growth of bacteria, this 

 acid has no appreciable action, especially in small quantity, on the un- 

 organised ferments. Pepsin is much more rapidly destroyed by standing 

 under strong alcohol than are the other enzymes. 



Anything which prevents swelling of the proteid by the acid retards 

 the progress of peptic digestion. Briicke 2 states that fibrin tied tightly 

 round with a thread, so that it cannot be so easily swollen by the acid, 

 is digested much more slowly. Adding a sufficient amount of neutral 

 salt also slows the digestion, probably from a similar cause, the salt 

 preventing the imbibition of acid by the fibrin. The comparative slow- 

 ness of digestion of heat-coagulated proteid, such as coagulated white of 

 egg, may also be due to a like cause, for such a form of proteid does not 

 swell up with acid. Finally, stronger acid than the optimum strength 

 does not cause so much swelling, and this may in part be the reason of 

 the slowing due to this cause. 



Variation in rapidity with form of proteid. The time of digestion 

 by pepsin varies enormously with the nature and condition of the pro- 

 teid to be digested ; coagulated white of egg requires almost as many 

 hours as unboiled fibrin does minutes. The comparative rate of pepton- 

 isation of coagulated and non-coagulated white of egg has been much 

 investigated, and with varying results. According to Waurinski, 3 these 

 variations are due to want of uniformity in the concentration of acid 

 employed as a digesting medium ; with more dilute acid the coagulated 

 proteid is much more quickly digested, but the reverse is true when acid 

 of greater concentration is used. 



The comparative speed of peptic digestion of different kinds of pro- 

 teid has, because of its practical bearing, been made the subject of much 

 investigation. 4 



Casein is the most easily digested of all forms of proteid. Fibrin is 

 much more quickly digested than coagulated egg-white, though, according 

 to its state of aggregation and time of boiling, the latter shows a con- 

 siderable variation. In general, proteids of animal origin are more 

 easily digested than those of vegetables, and of the latter legumin is 

 most easily, glutin most difficultly, digestible. 5 Jessen 6 observed that 

 muscle fibre is more rapidly dissolved when raw than when coagulated 

 by boiling or roasting, and that boiled milk is digested more slowly than 

 unboiled. Beef appears to be both more easily dissolved and peptonised 

 than fish. 7 



The conclusion ought not, however, to be too hastily drawn that 

 those forms of proteid which are most easily dissolved by gastric juice 

 are therefore best and most nutritious; gastric juice is not the only 

 proteolytic fluid which acts on the food. If the food has been properly 



1 Verhandl. cL naturh.-med. Ver. zu Heidelberg, 1876. 1ST. F., Bd. i. S. 90. 



2 " Voiiesungen," Wien, 1887, Aufl. 4, Th. 1, S. 312. 



3 Jahresb. u. d. Fortschr. d. Thier-Chem., Wiesbaden, 1873, Bd. iii. S. 175. 



4 Besides those quoted below, see Stutzer, Ztschr. f. physiol. Chem., Strassburg, 1885, 

 Bd. ix. S. 212 ; 1886, Bd. x. S. 153 ; 1887, Bd. xi. S. 207 ; 1888, Bd. xii. S. 72 ; Pfeiffer, 

 ibid., 1887, Bd. xi. S. 1; Wolff, Landwirthsch. Jahrb., 1890, Bd. xix. S. 795; Hahn, 

 Vircliows Archiv, 1894, Bd. cxxxvii. S. 597. 



5 Maly, in Hermann's " Handbuch," Bd. v. (2) S. 79. 



6 Ztschr. f. Biol., Miinchen, 1883, Bd. xix. S. 129. See also Bergeat, ibid., 1888, Bd. 

 xxiv. S. 1391 



7 Chittenden and Cummins, Am. Chem. Journ., Baltimore, 1884, vol. vi. No. 5 ; 

 Popoff, Ztschr. f. physiol. Chem., Strassburg, 1890, Bd. xxiv. S. 524. 



