RESISTANCE OF THE TISSUES 283 



Saprophytic bacteria of various kinds are normally present on 

 exposed surfaces of skin or mucous membrane. Tissues, also, which 

 are dead or depressed in vitality from injury or previous disease, 

 but which are still in contact with the living body, afford an excellent 

 nidus for the growth of bacteria. Still these have not the power, 

 unless specific, to thrive in the normal living tissue. It has been 

 definitely shown that the natural fluids of the body have in their 

 fresh state protective substances (alexines) which prevent bacteria 

 from flourishing in these tissues. Such protection depends in measure 

 upon the number of invading germs as well as their quality, 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 phagocytosis, 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. Eings of lymphoid tissue surround the oral 

 openings of the trachea and oesophagus, and the tonsils are masses 

 of lymphoid tissue. Composed as it is of cells having a germicidal 

 influence when in health, the lymphoid tissue may afford .formidable 

 obstruction to invading 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 disease-producing germs. But we know from experi- 

 ence that a full measure of health is not often the happy condition 

 of human tissues. There are a variety of circumstances which 

 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. Again, antecedent disease predisposes the tissues to 

 form a nidus for bacteria, and conditions of environment or personal 

 habits act powerfully in the same way. Damp soils must be held 

 responsible for many disasters to health, not directly, but indirectly, 

 by predisposition ; dirty houses and insanitary houses, dusty trades 

 and injurious occupations, have a similar effect. Any one of these 

 different influences may in a variety of ways affect the tissues and 

 increase their susceptibility to disease. Not infrequently we may 

 get them combined. For example, the following is not an unlikely 

 series of events terminating in consumption (tuberculosis of the 

 lungs): — (a) The individual is predisposed by inheritance to 

 tuberculosis; (6) an ordinary chronic catarrh, which lowers the 

 resisting power of the lungs, may be contracted ; (c) the epithelial 

 collections in the air vesicles of the lung — i.e. dead matter attached 

 to the body — afford an excellent nidus for bacteria; (d) owing to 

 occupation, or personal habits, or surroundings, the patient comes 



