GENERAL STRUCTURE 



23 



order to continue their existence. Therefore a method of pre- 

 paring the food for the use of the cells, and a way of trans- 

 porting it to each of them are necessary. 



The former is accomplished by the digestive system, which, 

 through the agency of the salivary, gastric, pancreatic, and 

 other glands, transforms the food into a special liquid state 

 capable of being absorbed by the millions of minute finger-like 

 villi of the small intestine (Fig. 71a). Thence it is transferred 

 by the lacteal vessels and veins to the heart, whence it is con- 



FiG. 12. — Subcutaneous Tissue from a Cat. (Lewis and Stohr.) 

 The fiber a has been treated with dilute acetic acid; the other fibers have 

 been teased apart and examined, unstained, in water, a, c. White fibers; h, 

 fat cell; d, connective tissue cell; e, elastic fibers. 



veyed by the arteries to their capillary distribution in the neigh- 

 borhood of every cell in the body. 



The waste material resulting from the cell activity is of two 

 kinds, gaseous and liquid. These waste products diffuse 

 through the capillary walls into the blood or are carried through 

 the lymphatics to the veins. Carbon dioxid is carried to the 

 heart and thence by the pulmonary arteries to the lungs, where 

 it passes into the terminal branches of the trachea, and finally 

 by expiration reaches the exterior. The liquid excretion is 

 transported by means of the lymphatics, veins and arteries to 

 the kidneys, where it diffuses through the walls of the capillaries 

 into the minute tubules opening into the ureters leading to the 



