GENERAL STRUCTURE AND COMPOSITION 15 



stances, and they are not themselves necessarily used up or de- 

 stroyed in the process. Many enzyms of great physiological im- 

 portance exist in the digestive fluids and play a part in fitting food 

 for absorption from the alimentary canal. For example, pepsin 

 found in the gastric juice converts, under suitable conditions, such 

 complex proteins as albumins into simpler peptones; ptyalin, found 

 in the saliva, converts starch into sugar. We shall have occasion 

 later to study a number of enzyms more in detail in connection 

 with their physiological uses. A characteristic property of all 

 enzyms is their susceptibility to heat; a temperature of 60 C. 

 suffices to destroy them completely. 



Non-Nitrogenous Organic Compounds. These may be con- 

 veniently grouped as hydrocarbons or fatty bodies; carbohydrates 

 or amyloids; and certain non-nitrogenous acids. 



Fats. The fats all contain carbon, hydrogen, and oxygen, the 

 oxygen being present in small proportion as compared with the 

 hydrogen. Three fats occur in the Body in large quantities, viz. : 

 palmatin (CsiHosOe), stearin (C 5 iHiioO 6 ), and olein (C^JHiQ^Oo). 

 The two former when pure are solid at the temperature of the 

 Body, but in it are mixed with olein (which is liquid) in such pro- 

 portions as to be kept fluid. The total quantity of fat in the Body 

 is subject to great variations, but its average quantity in a man 

 weighing 75 kilograms (165 pounds) is about 2.75 kilograms (6 

 pounds). 



Each of these fats when heated with a caustic alkali, in the 

 presence of water, breaks up into a fatty acid (stearic, palmitic, or 

 oleic as the case may be), and glycerin. The fatty acid unites 

 with the alkali present to form a soap. 



Carbohydrates. These may be defined as substances composed 

 of carbon, hydrogen, and oxygen, having the number of carbon 

 atoms in each molecule usually six or some multiple thereof, and 

 having the hydrogen and oxygen present in the same proportion 

 as in water. The three chief groups are the sugars, starches, and 

 cellulose. 



Dextrose or grape sugar (CeH^Oe) is the most important representative of 

 the sugars found in the Body. A large part of the food supply is received 

 from the digestive tract into the blood in this form. It occurs constantly in 

 small concentration in the blood and tissues. 



Lactose, the sugar of milk, occurs in considerable quantity in milk. 



