50 DIGEST/OX 



Fats. These are compounds of higher fatty acids and 

 glycerip. Ordinary body fat consists of mixtures of three 

 neutral fats (pal matin, stearin, and olein). Olein melts at 

 0.5 C., while palmatin and stearin melt at temperatures 

 higher than body temperature. Fats are soluble in ether and 

 in hot alcohol, but not in water. 



Enzymes. In passing along the alimentary tract the 

 various foods or foodstuffs undergo a series of physical and 

 chemical changes, which, grouped together, constitute the 

 digestion of food. The effect of these changes is largely the 

 transformation of insoluble, indiffusible substances into simpler, 

 soluble, and diffusible substances. The chemical changes 

 are of a peculiar character and are due to the presence of 

 enzymes in the secretions which are poured into the alimentary 

 tract. Enzymes are bodies of unknown structure whose presence 

 is inferred from the changes which demonstrably take place 

 in the foods undergoing digestion. Their essential nature is 

 unknown. They are not necessarily protein, but many of them 

 give protein reactions. They are non-living bodies, although 

 derived from living bodies. They are believed to be colloids. 

 In some there is present sctme inorganic substance which is 

 essential to its action. Examples are hydrochloric acid loosely 

 combined in pepsin and manganese in the ferment laccase. 



Enzymes are present in the digestive secretions in very 

 small amounts and act with extraordinary energy, without 

 being themselves used up in this process. The old distinction 

 between organized ferments and unorganized ferments, or 

 enzymes, has lost significance since an increasing number of 

 organized ferments have yielded enzymes which do their 

 characteristic work when separated from living cells just as 

 well as when part of the living organism. A better classifica- 

 tion is into intracellular and extracellular enzymes, i. e., 

 those enzymes which normally act within or outside of the cells 

 which produce them. The digestive enzymes, on this basis, 

 are mainly extracellular. Taken altogether they exhibit a 

 number of general properties : 



(a) They are soluble in glycerin and in aqueous or saline 

 solutions. Some act best in alkaline, some in neutral, and 

 some in acid media. 



