268 BACTERIA 



shown that the blood fluids of the body have in their fresh 

 state the germicidal power {alexines) which prevents bacteria 

 from flourishing in them. Such action does undoubtedly 

 depend in measure upon the number of germs as well as 

 their quaHty, for the killing power of blood and lymph must 

 be limited. Buchner has pointed out that the antagonistic 

 action of these fluids depends in part possibly upon pha-, 

 gocytosis, but largely upon a chemical condition of the 

 serum. The blood, then, is no friend to intruding bacteria. 

 Its efforts are to a certain extent seconded by the lymphoid 

 tissue throughout the body. Rings of lymphoid tissue 

 surround the oral openings of the trachea (windpipe) and 

 oesophagus (gullet) ; the tonsils are masses of lymphoid 

 tissue. Composed as it is of cells having a germicidal in- 

 fluence when in health, the lymphoid tissue may afford 

 formidable obstruction to intruding germs. 



All the foregoing points in one direction, namely, that if 

 the tissues are maintained in sound health, they form a very 

 resistant barrier against bacteria. But we know from ex- 

 perience that a full measure of health is not often the happy 

 condition of human tissues ; we have, in short, a variety of 

 circumstances which, as we say, predispose the individual to 

 disease. One of the commonest forms of predisposition is 

 that due to heredity. Probably it is true that what are 

 known as hereditary diseases are due far more to a hereditary 

 predisposition than to any transmission of the virus itself in 

 any form. Antecedent disease predisposes the tissues to form 

 a nidus for bacteria; conditions of environment or personal 

 habits frequently act in the same way. Damp soils must be 

 held responsible for many disasters to health, not directly, 

 but indirectly, by predisposition ; dusty trades and injurious 

 occupations have a similar effect. Any one of these three 

 different influences may in a variety of ways affect the 

 tissues and increase their susceptibility to disease. Not in- 

 frequently we may get them combined. For example, the 



