The Animal Body. 



tern and utilized in building up or renewing the tissue of the 

 body. Hay and grain cannot directly be transferred to the 

 blood, but must first be brought into soluble and diffusible con- 

 dition before they can pass out of the alimentary tract into the 

 blood and lymph. This is accomplished partly by mechanical 

 means, but mainly by chemical changes, which are produced 

 chiefly by the action of bodies called enzymes. 



Enzymes are a peculiar class of substances produced by living 

 cells which constitute the various secreting glands. They are of 

 unknown composition and are peculiar in that the chemical 

 changes which they induce are the result of what is called cat- 

 alysis, or contact. That is, during the solution of the food stuffs, 

 the enzyme is not used up or destroyed, but by its mere presence 

 sets in motion or quickens a reaction between two other sub- 

 stances. For example, the enzyme of the saliva causes the starch 

 of the food to combine with water, with the result that the soluble 

 sugar maltose, is formed. An enzyme that acts upon starch, for 

 example, cannot act on proteins or fats. Some digestive fluids 

 have the power of producing changes in different classes of food 

 stuffs, but when this occurs, it is assumed to be due to the presence 

 in the same fluid of different enzymes. Again, enzymes are sen- 

 sitive to their environment, and a proper temperature and re- 

 action must be maintained for their activity. The activity of 

 saliva is extremely sensitive to the nature of the reaction and 

 ceases when that becomes acid. Enzymes are thus seen to be 

 more or less unstable substances, endowed with great power as 

 digestive agents, but sensitive to a high degree and working ad- 

 vantageously only under definite conditions. 



Digestion in the mouth. The first step is mastication, by 

 which the food is subdivided and crushed by the action of the 

 teeth and thoroughly mixed with saliva. This special secretion 

 has its origin in several secreting glands, and from these this 

 liquid is poured into the mouth through ducts, opening in the 

 cheek under the tongue. Saliva is a highly dilute liquid of faint- 

 ly alkaline reaction and contains an enzyme, ptyalin, which has 



