314 A MANUAL OF PHYSIOLOGY 



II. The Chemical Phenomena of Digestion. 



Ferments. The chemical changes wrought in the food as it 

 passes along the alimentary canal are due to the secretions of 

 various glands which line its cavities or pour their juices into it 

 through special ducts. These secretions owe their power for the 

 most part to substances present in them in very small amount, 

 but which, nevertheless, act with extraordinary energy upon the 

 various constituents of the food, causing profound changes with- 

 out, upon the whole, being themselves used up, or their digestive 

 power affected. The active agents are the enzymes, sometimes 

 spoken of as unformed or unorganized ferments unorganized 

 because their action does not depend upon the growth of living 

 cells, which was long supposed to be the case for some other 

 ferments, such as yeast. Since it has been shown that specific 

 enzymes can be separated from cells which were formerly believed 

 to act by their mere growth, the distinction between formed and 

 unformed ferments has lost its significance, and has to a great 

 extent been superseded by the distinction between intra- and 

 extra-cellular enzymes i.e., between ferments which normally 

 act in the interior of the cells where they are produced and 

 ferments which act outside of the cells that secrete them. From 

 yeast cultures, for instance, by crushing the cells, a substance 

 can be obtained which in the complete absence of living yeast- 

 cells, and, indeed, of any living micro-organism, forms alcohol 

 and carbon dioxide from sugar, just as living yeast does. There is 

 every reason to believe that it is by the intracellular action of this 

 endoenzyme that the yeast-cell normally causes alcoholic fermen- 

 tation. The digestive ferments are typical extracellular enzymes. 

 Their chemical nature has not been exactly made out ; some of 

 them at least do not appear to be proteins, or to contain a protein 

 group. Some of them apparently exist in the colloidal condition, 

 although this has not been shown for all. In certain cases the 

 more or less stable union of a definite inorganic substance with the 

 ferment, or its actual inclusion in the ferment molecule, seems to 

 be a condition of its action. Thus there is reason to believe that 

 in gastric digestion hydrochloric acid is loosely combined with 

 the pepsin. In the plant oxydase, laccase (p. 264), manganese 

 is present. And the fact that manganese salts oxidize certain 

 substances as laccase does suggests that it is the manganese in 

 combination with some protein or other organic compound in 

 the ferment molecule which confers upon laccase its oxidizing 

 power. A similar relation between iron and some animal oxydases 

 is possible, though not definitely proved. But none of the 

 ferments of the digestive juices has as yet been satisfactorily 

 isolated, and at present it is only by their effects that we recognise 



