MATERIAL WITHIN THE BODY i8i 



ever is characteristic or distinctive of the plasma of an in- 

 dividual or of a species will be found in the serum. 



150. The lymph. The blood is confined to a set of tubes from 

 which it cannot escape— as blood. The whole system is there- 

 fore called a closed system. Outside the blood vessels, filling 

 definite tracts as well as spaces between tissue masses and cells, 

 is a colorless liquid called the lymph. It is jrom the lymph that 

 the cells obtain their food supplies, water, salts, ferments, and 

 oxygen ; and it is to. the lymph that they discharge their carbon 

 dioxid, urea and other wastes, and any special substances that 

 they may secrete. There is a definite connection between 

 lymph spaces and certain large blood vessels. The main com- 

 munication between the lymph and the blood is by osmosis 

 through the smallest blood vessels (see Fig. 93). The lymph, 

 like plasma or serum, consists chiefly of water, and carries prac- 

 tically the same kinds of substances in solution. In addition 

 the lymph has many white corpuscles floating in it, so that it 

 may be considered the same as blood but lacking red corpuscles. 



151. The heart and the vessels. The blood is kept moving 

 by the rhythmic contractions of the pumping organ, the heart. 

 Blood comes into the heart through vessels which are called 

 veins ; blood flows out of the heart in tubes known as arteries. 

 The arteries branch and divide again and again, reaching all 

 parts of the body. The smallest branches, the capillaries, form 

 a network, combining into larger and larger tubes and bringing 

 the blood over from the arteries to the veins. 



Among warm-blooded animals (birds and mammals) the heart 

 is a double organ: blood cannot pass directly from the right 

 half to the left half. Each half of the heart consists of an upper 

 receiving chamber and a lower pumping chamber (see Fig. 95). 



The left heart is somewhat larger and stronger than the right 

 heart. Its ventricle, or pumping chamber, closes up, or con- 

 tracts, at fairly regular intervals, forcing the contained blood 

 into the largest artery of the body, the aorta. The branches of 

 the aorta carry the blood on to the various organs and tissues. 

 The auricle, or receiving chamber, of the left heart is connected 



