THE BLOOD OF ANIMALS. 99 
All blood-corpuscles are microscopic. The white are 
more uniform in size than the red; and generally smaller 
(except in Mammals), being about 3355 of an inch in di- 
aieter. The red corpuscles are largest in Reptiles (those 
of the amphibious Proteus being the extreme, or s}5 of 
an inch), next in Fishes, then Birds and Mammals. The 
smnallest known are those of the Musk-deer. In Mammals, 
the size agrees with the size of the animal only within a 
natural order; but in Birds the correspondence holds good 
throughout the class, the largest being found in the Os- 
trich, and the smallest in the Humming-bird. In Man, 
they measure z355 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 glob- 
ular, or sac-like, in all animals; but they are constantly 
changing. 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, 
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Fig. 64.—Comparative Size and Shape of the red Corpuscles of various Animals. 
