RESISTANCE AGAINST BACTERIA. 33! 



the answer appears to be, briefly, that the living tissue pro- 

 duces certain active chemical substances which serve as germi- 

 cides, actually destroying the living bacteria which get into 

 the tissue, or at least preventing their growth ; further, that 

 the parasitic bacteria alone are able to overcome the deleterious 

 action of these substances. 



One phase of the matter is of the greatest importance to our 

 subject. This resisting power of the living tissue against the 

 invasion of bacteria has its influence upon the true parasites as 

 well as upon other bacteria, only its repressing influence is 

 much less, and indeed so slight that it may frequently be 

 overcome by the invaders. This resistance is exerted against 

 all species of bacteria. Against the common saprophytes it is 

 perfectly efficient ; against some parasitic bacteria it is moder- 

 ately efficient and will, in many cases, prevent the development 

 of the disease, even after the parasitic bacteria have entered 

 (tuberculosis). Against other bacteria the resisting power is 

 extremely slight (anthrax). The resisting power varies with 

 different species of animals, some species having the power of 

 absolutely resisting certain bacteria, when we call them immune. 

 Man is immune against hog cholera while the hog is not. 

 Lastly, the resisting power varies with the individual. Some 

 members of a species will have the resisting power highly de- 

 veloped, while others yield easily to invasion. This we speak 

 of as individual resistance. 



Now this resisting power is clearly located in the living cells 

 of the body and is dependent upon their normal functions. It 

 is only the living cell which can resist the invasion of micro- 

 organisms, either wholly or partially. From this it follows 

 that the resistance will be greatest when the body cells are in 

 the highest state of physical activity, and will diminish when 

 they become somewhat impaired in vitality. Anything, then, 

 which tends to reduce the physical health of the individual 

 tends to reduce his power of resistance. For example, some- 



