CIRCULATORY AND LYMPHATIC SYSTEMS 279 



the middle of the blood-stream, moving along more rapidly than 

 the white ones. 



422. The White Corpuscles (or leucocytes, as they are also 

 termed) are protoplasmic cells, and have no cell-wall, and 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 (called phagocytes) have 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. 



423. The disease- producing germs above mentioned — microbes 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 overrun the system, and destroy man- 

 kind 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, etc. 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 dissemination. 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. 



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

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



