26 



ZOOLOGY 



are the streets through which traffic has to pass, in 

 order that every part may be served with the food and 



The leuco- 

 cytes or 

 white cells 



Drawing by W. P. Hay. after Perrier 



FIG. 6. Blood corpuscles of various animals, a, human red blood corpuscles; 

 b, red and white corpuscles of the pigeon; c, red corpuscles of a frog; d, red and 

 white corpuscles of a snake ; e, red corpuscles of Proteus ; /, colorless corpuscles of 

 a sphinx caterpillar ; g, colorless corpuscle of a river mussel. All magnified to the 

 same degree. 



oxygen necessary for life, and the waste materials may 

 be removed. The principal cells of the blood are known 

 as the red and white corpuscles, the former vastly more 

 numerous. The red cells, which appear pale yellowish 

 when seen singly, are the carriers of oxygen. The white 

 cells, really colorless rather than white, are capable of 

 motion in the manner of simple Protozoa. They have 

 been called the policemen of the blood, because they 

 attack and devour injurious bacteria and other particles. 

 They are more efficient perhaps than the policemen of 

 our streets, since they execute sentence and effectively 

 dispose of the criminal at the moment of making the 

 arrest. This process is called phagocytosis, and is re- 

 garded as one of the important ways of protecting the 

 body from disease. It is, however, less important than 

 was formerly supposed, since the blood-fluid (serum) it- 



