THE RED CORPUSCLES OF OTHER ANIMALS. 177 



by far the most numerous and give the blood its color; they 

 are so tiny and so plentiful that about five millions of them 

 are contained in one small drop of blood. They are so 

 closely packed that the unaided eye cannot see the spaces 

 between them, and so the whole blood appears uniformly 

 red. 



The Red Corpuscles of Human Blood (Fig. 53) are cir- 

 cular disks a little hollowed out on each face. Seen singly 

 with a microscope each is not red but pale yellow; it is 

 only when they are crowded in a heap that the mass looks 

 red; a drop of blood spread out very thin on glass, or mixed 

 with a tablespoonful of water, is pale yellow and not red. 

 Soon after blood is drawn most of the red corpuscles cohere 

 side by side in rows, something like piles of coin. 



The Red Corpuscles of other Animals. The red corpus- 

 cles of most mammalia resemble those of man in being 

 circular biconcave pale yel- 

 low disks; those of camels 

 and dromedaries, however, 

 are oyal. The blood-cor- 

 puscles of dogs are so 

 like those of man in size 



that they Cannot be Fia. 54.-Red corpuscles of the Frog. 



readily distinguished; but in most cases the size is suffi- 

 ciently different to enable a safe opinion to be formed. This 



Which kind is most numerous? Give some idea of their num. 

 ber. Why does the blood look uniform!}'' red to the unaided eye? 



Describe the form of human red blood-corpuscles. What is 

 the color of one seen by itself with a microscope? How may we 

 show that blood looks red only when its corpuscles are crowded close 

 together? How do the red corpuscles become arranged soon after 

 blood is drawn ? 



Describe the corpuscles of most mammalia. How do those of 

 camels and dromedaries differ from the corpuscles of other mam- 

 mals? Why cannot a dog's blood be easily distinguished from 

 human blood? 



