386 APPLIED PHYSIOLOGY 



687. How bacteria enter the body. Germs of an infec- 

 tious disease may remain alive outside the body in anything 

 containing animal or vegetable matter. They may grow 

 in damp clothing, or in sinks or wells, or in the soil, and 

 may cause disease in whoever happens to fake them into 

 the body. Dirt and filth make good soil for the growth of 

 the germs, and are well-known causes of disease. The 

 germs may become dry and remain in a dormant state for 

 years, and finally produce the disease again. 



Bacteria may enter the body wherever the epithelium is gone, and 

 the lymph spaces are bare. Even a scratch or a pin prick may admit 

 thousands at one time. They can also enter through the mucous mem- 

 brane of the nose or throat, or they may be swallowed in drinking 

 water or licked off from a knife or spoon which another person has used. 



688. How the body destroys bacteria. Germs of disease 

 constantly surround us, and the skin is constantly being 

 scratched and pricked, affording them entrance. Yet only 

 in rare instances do they grow and produce sickness, for 

 the body has three very efficient weapons of defense. 



First. The white blood cells have a special power of 

 seeking out bacteria and the toxins which they produce, 

 and of enveloping and destroying them (p. 397). 



Second. The plasma of the blood and lymph, by some 

 chemical power, is able to destroy germs of disease. 



Third. The serum of the blood often contains a sub- 

 stance called an antitoxin, which destroys the toxins of the 

 germs and so stops their action. When a disease has pro- 

 gressed for a few days or weeks, the antitoxin is formed in 

 sufficient amount to overcome the germs, and so the dis- 

 ease comes to an end. 



If the toxins of diphtheria germs grown outside the body are injected 

 into a horse, the plasma of its blood will contain the antitoxin of the 

 disease. If its blood is drawn and allowed to clot, the clear serum will 



