90 TRANSPORTATION SYSTEM IN ANIMALS 



branches whose subsequent union finally results in one 

 or two large vessels opening directly into a large vein in 

 the region of the shoulder. The lymph stream, diffusing 

 from the blood and flowing past the cells, enters these 

 lymph capillaries and is finally returned to the blood. 



Summary. — A transportation system, in the form of 

 vessels or extensive blood spaces, carries nutritive ma- 

 terials to and removes waste products from every body 

 cell in all but the simplest species of animals. In the 

 higher types the blood does not come into direct contact 

 with the tissues, but a constant stream of fluid, the lymph, 

 diffuses from the capillaries into adjoining spaces from 

 which the cells absorb their food materials and into which 

 they discharge their wastes. Beyond this point the 

 stream enters communicating lymph capillaries, and 

 slowly journeys through these into larger vessels that 

 finally pour their contents into the blood stream. We 

 shall now consider the method whereby food substances 

 are introduced into the transportation system and the 

 wastes are removed. 



