DIGESTION AND ABSORPTION 



353 



Glands. In addition to the alimentary canal proper, we find a 

 number of digestive glands, varying in size and position, connected 

 with the canal. As we have already learned, a gland is a col- 

 lection of cells which takes up materials from within the body 

 and pours out this material as a secretion. An example of glands 

 in plants is found in the nectar glands of a flower. 



Certain substances called enzymes formed by glands cause the 

 digestion of food. The enzymes secreted by the cells of the glands 

 and poured out into the food 

 tube act upon insoluble 

 foods so as to change them 

 to a soluble form. 



Structure. The entire 

 inner surface of the food 

 tube is covered with a soft 

 lining of mucous membrane. 

 This is always moist be- 

 cause 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 become indented at 

 this point to form a pitlike 

 gland. Often such depres- 

 sions are branched, thus 

 giving a greater secreting surface, as is seen in the Figure. 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 system. Think of a sour pickle and note what 

 happens. 



Attached to the digestive tract of man are found, besides the 

 salivary glands in the mouth, gastric glands in the walls of the 

 stomach, the liver and the pancreas, two large glands which empty 



HUNT. E8. BIO. 23 



Diagram of a gland : t, the common tube which 

 carries off the secretions formed in the cells 

 lining the cavity c; a, arteries carrying blood 

 to the glands; v, veins taking blood away from 

 the gland. 



