singly, are of a yellow colour, and float in the middle of the blood 

 stream, moving along more rapidly than the white ones. 



420. The White Corpuscles (^or /f//cori'^fs, as they are also termed), 

 are very remarkable bodies, which float more slowly along the sides 

 of the vessels, in what is called the still stream. They are larger than 

 the red corpuscles, and seem to have a sort of life in themselves. It 

 is believed that some of them {ca.\led phagocytes)ha.ve the power of killing 

 the disease germs which attack the body. There is still some doubt 

 as to the origin of the white corpuscles ; it is thought by some that 

 they are formed in the lymphatic glands ; by others, that they are 

 made in the spleen ; while others think that they come from the 

 middle parts, or medulla, the marrow of bone. 



421. The disease-producing germs above-mentioned — microhes, or 

 bacilli — surround us in millions, and enter the body by food, water, and 

 air. Were it not for the watchful guard which these little soldier- 

 like corpuscles keep, ready to pounce upon any intruders, these germs 

 would speedily over-run the system, and destroy mankind and animals 

 wholesale. Occasionally, indeed, when the body, from some cause or 

 other, has become relaxed, and the phagocytes are unable to do their 

 work properly, the disease-producing germs get the upper hand, and 

 set up their own particular disease, as, for example, scarlet fever, &c. 

 When once these germs get a footing in a human being, or in an 

 animal, they develop and become more numerous, and capable of 

 wide dissemmation. After a time, they appear to lose their potency, 

 and the bodies through which they have passed seem to be rendered 

 immune, and freed from subsequent attack. The epidemic dies out, 

 but the germs remain latent in our midst, until, in the course of time, 

 they seize hold of another susceptible subject, and again re-establish 

 their virulence. At least, from the various periodical outbreaks of 

 specific diseases, and the immunity given by inoculation, one is led to 

 think such is the case. 



422. Proteids. — The proteids of blood are plasma, fibrinogen, 

 serum albumin, serum globulin, and fibrin ferment ; of these fibrinogen, 



X 



