104 



GENERAL ZOOLOGY 



wall of the alimentary canal into the blood or lymph streams. 

 Liquefaction is produced by breaking down the complex organic 

 food substances into simpler compounds; but a chemical trans- 

 formation is also undergone by a soluble substance like cane 

 sugar. The diet of a frog or man consists of organic and inorganic 

 substances. Of the latter, water is the principal one taken alone. 

 Inorganic salts are of course present in meat, milk, vegetables 

 bread, etc., but we ordinarily think of these articles of food as 

 organic in nature. Since in the process of digestion inorganic 

 salts are released from organic combination and absorbed or 



Fig. 70. — Section of an island of Langerhans surrounded by glandular alveoli 

 of pancreas; diagrammatic, a, alveoli of glands drained by pancreatic duct; 

 b, blood vessel; i, island of Langerhans (islet cells), having no connection with 

 ducts. (After Stiihr.) 



secreted without further change, digestion is primarily concerned 

 with the changes undergone by the organic constituents of food. 

 The organic foods, protein, carbohydrate, and fat, are distinctly 

 different chemical substances, yet digestion in the case of each 

 is brought about by the same sort of chemical process, known as 

 hydrolysis, the essential feature of which is a preliminary union 

 with water and a subsequent splitting of the combination into 

 simpler products. An example of a relatively simple form of 

 hydrolytic cleavage may be illustrated by the breaking down of 

 a molecule of a polysaccharide, such as malt sugar, into two 

 molecules of a monosaccharide, dextrose: 



C12H22O11 + H 2 ■ 



malt sugar water 



2C6H12O6 



dextrose 



Digestion is not always completed by a single hydrolytic cleavage, 



