3 2o CHEMISTR Y OF THE DIGESTIVE PROCESSES. 
Rate of zymolysis l or enzyrnic action.— The rate at which diges- 
tion o-oes on in any digestive fluid varies chiefly with the following 
conditions, namely— (1) The temperature, (2) the reaction, (3) the con- 
centration of the products of digestion, (4) the concentration of the 
digestive enzyme, (5) the condition of the material to be digested. 
3 Temperature. — The digestive enzymes are very sensitive to changes 
in temperature ; they all act most energetically at or slightly above the 
body temperature. The point of greatest activity is called the optimum 
point ; as the temperature varies, either above or below this point, the 
rapidity of action of the enzyme slackens ; and, as the interval apart from 
the optimum point is increased, a point is finally reached at which the 
action of the enzyme is no longer appreciable. Any temperature 
markedly above that of optimum action slowly destroys the enzyme, 
and this destructive action in all cases becomes very rapid at tempera- 
tures varying (between 50° and 65° C.) with the particular ferment, the 
reaction of the fluid in which it is so heated, and the degree of its dilu- 
tion. 2 On the other hand, low temperatures, though they slow and 
finally stop ferment action, do not destroy the ferment ; this recovers its 
activity completely when the temperature is again raised, even though 
the temperature has been kept at - 5° C. for several hours. 3 
Reaction. — The variation in chemical reaction of the fluid in which 
they act has a similar effect on enzymes to that of variation in tempera- 
ture. For each of the digestive enzymes there is a particular reaction, 
and degree of that reaction, at which it acts with maximum power. A 
departure from this degree of acidity or alkalinity causes a more or less 
rapid diminution in the speed with which the enzyme acts, and a 
sufficient amount of departure from the optimum reaction causes the 
destruction of the enzyme. Some of the enzymes act in solutions of 
either acid, neutral, or alkaline reaction, provided always that the 
reaction does not stray too widely from that at which they act best ; 
examples of such are ptyalin and trypsin. Others only act with one 
specific reaction, and are rapidly destroyed if the reaction changes from 
this. Examples of these are pepsin, only active in acid solution, and 
rapidly destroyed by a trace of alkalinity ; and the fat-splitting ferment 
of the pancreas, active only in alkaline or neutral solutions, and rapidly 
destroyed by acid. 
Accumulation of dissolved products of action. — Accumulation of the 
products of the action of an enzyme in the solution acts unfavourably 
upon its continued action, slowing and finally altogether checking it. 4 
This action may be to some extent prevented by removing the products 
formed by dialysis, or diluting them by the addition of water. In the 
latter case, however, the ferment is also diluted, and in the former, since 
the products of digestion in most cases have no very high diffusive 
power, the removal is very slow and incomplete. 
Eemoval of the digestive products by dialysis has, in addition, the 
disadvantage that the digestive solution is diluted by the osmosis, due to 
1 This term is that proposed by Sheridan Lea, Journ. Physiol., Cambridge and London, 
1S90, vol. xi. p. 254. 
- v. Wittich, Arch./, d. ges. Physiol., Bonn, 1869, Ed. ii. S. 193 ; 1S70, Bd. iii. S. 339. 
3 Bidder u. Schmidt, "Die Verdauungssafte, etc." 
4 Briicke, Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1862, Bd. xliii. S. 601 ; 
"Vorlesungen," Wien, 1885, Bd. i. S. 312; Cohnheim, Virchovfs Archir, 1863, Bd. 
xxviii. S. 241 ; Kiihne, " Lehrbuch der pliysiol. Chem.," 1866, S. 39, 51, 52 ; Sheridan Lea, 
Journ. Physiol., Cambridge and London, 1890, vol. xi. p. 226. 
