Page:Popular Science Monthly Volume 19.djvu/478

462 and the class of birds. The largest are found in the amphibians, those of the proteus being of an inch in diameter. The smallest are found in the musk-deer, being of an inch. Those of the ostrich are of an 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 to  inch. The value of microscopic measurements of blood-corpuscles, as evidence in legal cases, has been



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 corpuscles. 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 according to changes in the body or blood, and may bear a proportion to the red of one in one thousand to one in three hundred. Although generally globular, they have no fixed shape, but have amœboid movements. Indeed, the resemblance is so close between the amœba and these white corpuscles, that Professor Huxley, in defining the amœba, says it is structurally "a mere colorless blood-corpuscle leading an independent life." And again he says: "Leaving out the contractile vacuole, the resemblance of an amœba in its structure, manner of moving, 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