48 



APPLIED BIOLOGY 



chiefly prepared for absorption by being made into a 

 solution in water. Even liquid foods like milk and soup 

 must be dissolved so that no solid particles can be seen when 

 they are examined with a microscope. 



44. Digestion. Some solid foods, such as sugar and 

 salt, readily dissolve (are soluble) in water ; but most of the 

 frog's food consists of meat and other things which have 

 to be acted upon by certain substances before they will 

 dissolve in the water which is taken into the food-tube. 

 To this process of changing foods and causing them to dis- 

 solve the term digestion is given. Definition : Digestion is 

 the preparation of foods for absorption by the cells. It is 

 chiefly a changing of foods so that they dissolve in water. 



In order to understand how a chemical change may make 

 an insoluble substance capable of dissolving (soluble) in 

 water, try the following experiment : 



(D) Place a very small piece of marble or limestone (or a piece 

 of zinc) in water. Does it dissolve? 

 Pour some dilute hydrochloric acid into 

 the water, and repeat from time to time 

 until the limestone (or piece of zinc) 

 becomes dissolved in the water. 



From such experiments we learn 

 that some substances which are not 

 soluble in water will become soluble 

 after a chemical change has been 

 produced by an additional sub- 

 stance. This is just what happens 

 in the alimentary canal of the frog. 

 Small pocket-like tubes on the inside 

 wall or lining of the stomach and in- 

 testine (called respectively gastric 

 (stomach) and intestinal glands, 

 Fig. 18), as well as the liver and 

 pancreas, form or secrete peculiar 



FIG. 18. Lining membrane of 

 frog's stomach, g, gastric 

 gland composed of epi- 

 thelium; c, connective tissue; 

 m, muscle coat of stomach. 

 (From Ecker.) 



