34 NUTRITIONAL PHYSIOLOGY 



Enzymes are exceedingly sensitive to varying degrees of 

 acidity and alkalinity in the medium. Most of them do 

 not ]$:eep their efficacy if the solution is far from the 

 neutral point. But they are somewhat individual in this 

 as in other properties, the acid which is highly favorable 

 for gastric digestion, for example, being quite prohibitive 

 of salivary action. They are all destroyed when in solu- 

 tion by temperatures somewhat short of boiling. Cold 

 ^uspends their activity, but does not prevent its return 

 upon warming. They are most effective at a temperature 

 .not far from that of the blood, though in general a few 

 degrees higher. These relations between the enzymes 

 and temperature are much like those established in the 

 case of the simpler living forms. Having this in mind, 

 one easily adopts the common practice of speaking of the 

 killing of enzymes by heat. It must not be forgotten that 

 this is a figurative expression. We are not justified in 

 thinking of enzymes as living. Living organisms when 

 they grow and multiply in a nutrient medium may de- 

 compose it much as suitably assorted enzymes would do, 

 and, in fact, the organisms in question are probably pro- 

 ducing their own enzymes for the purpose. Formerly 

 such living things as the yeasts and the bacteria were de- 

 scribed as " organized ferments," and the detached en- 

 zymes, incapable of self-multiplication, were called 

 " unorganized ferments." These terms are not much used 

 at the present time. Enzymes are assumed to be products 

 of living cells and may be very characteristic fragments of 

 the cell's fabric, but they are not independently living. 



The digestive changes to which we pay most attention 

 are those which occur in the cavity of the alimentary 

 canal, and which can be observed to take place also when 

 the same mixtures are placed in the flasks and test-tubes 

 of the laboratory. But we must not overlook the prob- 

 able fact that similar changes are constantly occurring 

 within the boundaries of every active cell. Intracellular 

 digestion, presumably made possible by intracellular en- 

 zymes, obviously takes place when a protozoan cell engulfs 



