DIGESTION AND ABSORPTION 299 



stomach because of the change 4 from an alkaline surrounding in the 

 mouth to that of an acid in the stomach. Enzymes seem to be 



/ 



able to work indefinitely, providing the surroundings are favorable. 

 A small amount of digestive fluid, if it had long enough to work, 

 could therefore digest an indefinite amount of food. 



Gland Structure. - The entire inner surface of the food tube 

 is covered with a soft lining of mucous membrane. This is always 

 moist because certain cells, called mucus cells, empty out their 

 contents into the food tube, thus lubricating its inner surface. 

 When a large number of cells which have the power to secrete 

 fluids are collected together, the surface of the food tube may be- 

 come indented at this point to form a pitlike gland. Often such 

 depressions are branched, thus giving a greater secreting surface, 

 as is seen in the figure on page 298. The cells of the gland are 

 always supplied with blood vessels and -nerves, for the secretions 

 of the glands are under the control of the nervous sj^stem. 



How a Gland Secretes. - - We must therefore imagine that as the 

 blood goes to the cells of a gland it there loses some substances 

 which the gland cells take out and make over into the particular 

 enz} r me that they are called upon to manufacture. Under certain 

 conditions, such as the sight or smell of food, or even the desire 

 for it, the activity of the gland is stimulated. It then pours out 

 its secretion containing the digestive enzyme. Thus a gland does 

 its work. 



Salivary Glands. - - We are all familiar with the substance 

 called saliva which acts as a lubricant in the mouth. Saliva is 

 manufactured in the cells of three pairs of glands which empty 

 into the mouth, and which are called, according to their position, 

 the parotid (beside the ear), the sub-maxillary (under the jawbone), 

 and the sublingual (under the tongue) . 



Digestion of Starch. - - If we collect some saliva in a test tube, 

 add to it a little starch paste, place the tube containing the mixture 

 for a few minutes in tepid water, and then test with Fehling's 

 solution, we shall find grape sugar present. Careful tests of the 

 starch paste and of the saliva made separately will usually show 

 no grape sugar in either. 



If another test be made for grape sugar, in a test tube containing 



