DIGESTION 



233 



Salivary 

 Glands ^ 



animals or plants. The original source of all animal and 

 plant food, as has been seen, is in the chlorophyll manu- 

 factory of the leaf and green stem. After this leaf food 

 has been manufactured, it is simply modified by the plants 

 and animals through which it passes. The food is used 

 (1) in growing new cells, (2) in repairing cells that have 

 been used up or destroyed, (3) in providing energy to 

 carry on the activities of the body and maintain its heat 

 or (4) in doing external 

 work, such as moving the 

 body itself from place to 

 place or moving other 

 bodies. 



To furnish any of this 

 energy, the cells must be 

 able to combine food with 

 oxygen. To do this the 

 food must be digested or 

 prepared so that it can 

 pass through animal tis- 

 sue. In the higher 

 mals, a complicated ap- 

 paratus is provided to 

 accomplish this. In man, 

 it is briefly as follows : 

 a long continuous tube, the 

 food-tract or the alimen- 

 tary canal (Fig. 109), ex- 

 tends through the body. 

 Different portions of this tube are adapted to different 

 processes. In the mouth, the teeth grind the food into 

 small bits and mix it with the saliva. This is an exceed- 

 ingly important part of the process, because if the food is 

 not ground fine, the digestive juices cannot readily get at 



G&ll 

 am- Bladdc 



Fig. 109. 



