THE NATURE AND THE MEANS OF DIGESTION 33 



The enzyme in a digesting mixture is not forcibly compel- 

 ling the molecules to disintegrate, but it is removing some 

 hindrance to their spontaneous rearrangement. It is not 

 definitely used up in this service. Accordingly, it follows 

 that a limited quantity of a digestive juice, of course, 

 containing a still more limited quantity of enzyme, may 

 be responsible for an amount of digestion practically 

 unlimited. Unlimited time would be demanded for such 

 a demonstration. (This form of statement should be 

 qualified. Enzymes are somewhat unstable and liable 

 to deteriorate.) 



When a process of hydrolysis takes place under the in- 

 fluence of an enzyme and in a glass vessel, there must be a 

 rising percentage of the products and a declining per- 

 centage of the initial substance as the reaction goes on. 

 The velocity of the transformation is found to diminish 

 and at last it seems entirely arrested. A mixture now ex- 

 ists which contains the first and the last members of the 

 chemical system in proportions which have become con- 

 stant. It is an instance of chemical equilibrium. The 

 halting of the reaction does not mean that the enzyme is 

 exhausted. If any means can be devised by which the 

 accumulated end-products <;an be removed the hydrolysis 

 will be continued. It was specified above that the trial 

 should be made in a glass vessel. The reader will quickly 

 recognize the important difference between such a con- 

 tainer, from which nothing can escape, and the alimentary 

 canal, from which active absorption processes withdraw 

 the products of digestion. A clear field is thus provided 

 for the continuance to substantial completion of the reac- 

 tions which the enzymes are promoting. The contrast 

 between laboratory conditions and those which prevail 

 in the body did not escape the acute mind of Spallanzani, 

 who was a pioneer among students of these matters. As 

 early as 1777 he recorded that the solution of meat by 

 gastric juice could be greatly facilitated by letting the 

 digestive fluid fall drop by drop upon the food and to 

 trickle away, bearing the dissolved products. 



