The Body-Machine and Its Work 331 



we said, myriads of red corpuscles in a yellowish fluid. It was 

 found out some years ago that the serum is quite congenial to 

 its own corpuscles, but that if we mix into it a little of the blood 

 of some animal of a different kind, the serum of the first animal 

 destroys the red cells of the second animal. Thousands of experi- 

 ments were made, and it was found that the degree of action of 

 one kind of blood upon another depended on the nearness or 

 remoteness of relationship of the two animals. If they were 

 nearly related, there was no destructive action. Naturally, the 

 opportunity was soon sought to apply this new test of "man's 

 place in nature," and it was found that his blood mingled amiably 

 with that of the anthropoid apes! 



There is a third element in our drop of blood under the 

 microscope, and this is the most interesting of all. Here and 

 there in it, though hundreds of times less numerous than the red 

 disks, there are what are called "white corpuscles," microscopic 

 colourless roundish specks. When we study these specks closely, 

 we find that they behave just like the very primitive microscopic 

 animal known as the Amoeba. They push out parts of their sub- 

 stance, and glide along. If there are bacteria in the blood, one 

 of these corpuscles may be seen making its way to one of the 

 intruders and slowly folding its substance round it. After the 

 microbe is engulfed, digestion soon follows. 



In other words, there is in the blood, besides the army of 

 oxygen-carriers, a great army of defenders against bacteria. Let 

 a tissue be injured somewhere, and the injurious bacteria find a 

 footing and begin to multiply at an appalling rate. We are 

 threatened with disease, if not death. The bacteria may destroy 

 the tissue, or pour poison into the blood. But the "white knights" 

 now gather from all parts to defend the body. They are brought, 

 of course, by the flow of the blood, but they seem to have some 

 sort of chemical sense for bacteria, and they crowd in the par- 

 ticular tissue which is threatened. A great struggle ensues, and 

 the patient's temperature rises to "battle heat." If the white 



