PEPSIN. 333 
was formerly credited with a powerful checking action, but, as shown 
by Kuhne, 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 AVaurinski, 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 Verhaadl. d. natit.rJi.-med. Ver. zn. Heidelberg, 1876. N. F., Bd i S 90 
2 "Vorlesungen," Wien, 1887, Aufl. 4, Th. 1, S. 312.' 
3 Jahresb. ii. d. Fortschr. d. Tliier-Cliem., Wiesbaden, 1873, Bd. iii. S. 175. 
4 Besides those quoted below, see Stutzer, Ztschr.f. physiol. Chem., Strassbure 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, La-iidwirthsch. Jahrb., 1890, Bd. xix. S. 795; Halm' 
Virchoics Archie, 1894, Bd. exxxvii. S. 597. 
5 Maly, in Hermann's " Handbucb," Bd. v. (2) S. 79. 
6 Ztschr.f. Biol., Munchen, 1883, Bd. xix. S. 129. See also Bergeat, ibid., 1888, Bd. 
xxiv. S. 139. 
7 Chittenden and Cummins, Am. Chem. Journ., Baltimore, 1884, vol. vi. No. 5 • 
Popoff, Ztschr. f. physiol. Chem., Strassburg, 1890, Bd. xxiv. S. 524. 
