304 



COMPARATIVE ZOOLOGY 



only within a natural order ; but in birds the correspon- 

 dence holds good throughout the class, the largest being 

 found in the ostrich, and the smallest in the humming bird. 

 In man, they measure ^Vo of an inch, so that it would 

 take 40,000 to cover the head of a pin. 



As to shape, the colorless corpuscles are ordinarily 

 globular, in all animals ; but they are constantly chang- 



FIG. 262. Comparative Size and Shape of the red Corpuscles of various Animals. 



ing. The form of the red disks is more permanent, 

 although they are soft and elastic, so that they squeeze 

 through very narrow passages. They are oval, circular, 

 or angular, in fishes ; oval in reptiles, birds, and the 

 camel tribe ; and circular in the rest of mammals. They 

 are double convex when nucleated, and double concave 

 when circular and not nucleated. 



Blood is always heavier than water ; but is thinner in 

 cold-blooded than in warm-blooded animals, in herbi- 



