462 



THE POPULAR SCIENCE MONTHLY. 



mals and the class of birds. The largest are found in the amphibians, 

 those of the jyroteus being :^J ^ of an inch in diameter. The smallest 

 are found in the musk-deer, being j-^^q of an inch. Those of the 

 ostrich are y^Vo ^^ ^^ inch, and of the humming-bird ^^^ of an 

 inch. Yet this tiniest of vertebrates equals, in the size of its blood- 

 corpuscles, the largest of living creatures, the bulky whale. Those 

 of man are from -g-oVo ^^ t^W ^^ ^^ inch. The value of microscopic 

 measurements of blood-corpuscles, as evidence in legal cases, has been 



Fig. 3. Blood Corpuscles op the Frog ; magnified 370 diameters, showing the nucleus. 



much overrated. It is quite impossible to distinguish human blood 

 from that of the dog, and, without very extensive measurements, from 

 that of some other mammals. 



These red corpuscles are frequently larger than the capillary tubes 

 through which they have to pass, but, on account of their elasticity, 

 they squeeze through and afterward regain their shape. 



It is estimated that a drop of human blood contains one million 

 corpuscles a late authority says five millions in a cubic millimetre. 



In addition to the red corpuscles of the vertebrates, all true blood 

 contains colorless cori^uscles. These are nucleated in the vertebrates, 

 mollusks, and higher articulates. They are usually smaller than the 

 red, and not nearly so numerous. They vary rapidly in number accord- 

 ing to changes in the hqdy or blood, and may bear a proportion to the 

 red of one in one thousand to one in three hundred. Although gen- 

 erally globular, they have no fixed shape, but have amoeboid move- 

 ments. Indeed, the resemblance is so close between the amoeba and 

 these white corpuscles, that Professor Huxley, in defining the amoeba, 

 says it is structurally " a mere colorless blood-corpuscle leading an in- 

 dependent life." And again he says: "Leaving out the contractile 

 vacuole, the resemblance of an amoeba in its structure, manner of mov- 

 ing, and even of feeding, to a colorless corpuscle of the blood of one 

 of the higher animals is particularly noteworthy " ; also in a foot-note 



