vii THE DIGESTIVE SYSTEM AND ITS FUNCTIONS 137 



some form of proteid. They have the property of causing 

 chemical changes in other bodies without suffering any, or 

 at least but very little, destruction of their own substance. 

 A very minute amount of enzyme will cause the fermenta- 

 tion of a very large amount of other material. Near the 

 freezing point 'the action of enzymes is almost nil ; but with 

 increase of temperature their action goes on much more 

 rapidly until a maximum is reached beyond which further 

 increase of temperature checks the process. A temperature 

 of 100 C. destroys the action of ferments entirely. 



The substances which may serve as food are the proteids, 

 fats, carbohydrates, water, salts of various kinds, and a few 

 other substances not falling into any of these categories. 



The proteids are the most essential of the food materials, 

 since they contain in addition to the carbon, oxygen and 

 hydrogen found in carbohydrates and fats, the element nitro- 

 gen, and in many cases a certain number of other elements 

 besides. The white of egg, muscle, in fact most animal foods 

 with the exception of fat, consist largely of different forms 

 of proteid. In fats only carbon, oxygen, and hydrogen are 

 present, and the proportion of oxygen is small. Chemically, 

 fats are compounds of glycerin with some fatty acid. 



The carbohydrates are compounds of carbon, oxygen, and 

 hydrogen, the two latter elements being in the proportion in 

 which they occur in water ; in other words, there are twice 

 as many atoms of hydrogen as of oxygen in each carbo- 

 hydrate molecule. Sugar and starch are examples of this 

 class of food. 



All of these classes of food are acted upon by specific 

 ferments, which render them soluble and capable of diffusing 

 through the walls of the alimentary canal. The action of the 

 different digestive fluids will be described in connection with 

 the organs by which they are produced. 



