I8o ELEMENTARY BIOLOGY 



from various organs and tissues of the body. It is not to be 

 supposed that any given drop of blood will contain all of these 

 substances, or all of them in the same proportions, for, as we 

 shall see, the composition of the blood is constantly under- 

 going changes in the smallest blood vessels, through the 

 walls of which new substances are coming in and others are 

 passing out. 



210. The lymph. The blood, consisting of plasma and cor- 

 puscles, fills a set of tubes from which there are no openings ; 

 the system is therefore called a closed blood system, to distin- 

 guish it from the blood 

 system of clams and 

 certain other animals, 

 upj in which many of the 

 blood tubes have open 

 ends connecting with 

 various spaces in the 



FIG. 67. Human blood corpuscles ~ . , .. , 



body. Outside of the 



, fresh leucocytes (white corpuscles) in resting and ,, _, i f u h 



in moving stage (note granulations and nuclei) ; b, red 



corpuscles in flat and in side view man body, filling defi- 



nite tracts as well as 



spaces between tissue masses and cells, is a colorless liquid, 

 called lymph. It is from 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. The communication between the lymph and the 

 blood is by osmosis through the walls of the smallest blood 

 vessels (Fig. 68), and through definite connections between 

 lymph tubes and certain large blood vessels. 



The lymph, like the plasma, consists chiefly of water, and 

 carries practically the same kinds of substances in solution. 

 In addition, the lymph has floating in it many white corpus- 

 cles, so that it may be compared to blood lacking the red 

 corpuscles. The lymph has been compared to the ocean, in 

 which life probably originated, and from which so many 



