236 BACTERIA, YEASTS, AND MOLDS 



many little sores, festers, boils, and abscesses, all of which 

 are commonly due to bacteria entering through the skin 

 (Fig- 78)- These bacteria are harmless in the 

 stomach, and, indeed, we are swallowing them 

 all the time. The mouth contains great num- 

 bers of these bacteria, as well as numerous 

 other species, but they do us no injury. Skin 

 diseases like ringworm, favus, etc., enter in 

 the same way. This is likewise the case with 

 lockjaw (Fig. 79), erysipelas (Fig. 78, b), and 

 various forms of blood poisoning, some of 

 which are of comparatively little importance, 

 while others may be serious and fatal. It is 

 FIG. 78. Vari- possibly also true of measles, scarlet fever, 

 ous pat ho- and smallpox, although in these latter cases 

 gemc cocci. we nave re ally no knowledge of the matter. 



a, pus cocci; 



b cocci pro- But tnou gh these diseases enter through 

 ducingpneu- the skin, it should be remembered that the 

 monia; c, surface of the body is commonly quite well 

 protected against the invasion of microorgan- 

 isms. We have already seen that the skin of 

 fruits, if uninjured, protects the softer portion of the 

 interior from decay to a considerable extent, and that the 

 organisms which produce decay usually enter 

 through bruises, cracks, or cuts in the skin. 

 Precisely the same thing is true, probably to 

 an even greater extent, in the case of the FlG ?9 

 human body. The outer layer of the skin is a Bacillus of 

 protection which the bacteria cannot ordinarily tetanus 

 penetrate. If therefore the skin is unbroken and unin- 

 jured, a person is almost perfectly protected against the 



