82 MANUAL OF PHYSIOLOGY. 



may be regarded as physiological. It is their relation to the dis- 

 eased state that makes a knowledge of these creatures impera- 

 tive to medical men. 



So long as the tissue of a higher animal is healthy and well 

 nourished, bacteria cannot thrive in immediate contact with it. 

 They can only exist in the intestine, etc., because there they find 

 accumulations of lifeless fluids which offer them a suitable nidus. 

 Active living tissues have antiseptic power, i. e., are able to de- 

 stroy bacteria, and it is only owing to this bactericide power of 

 our textures, that we can with immunity breathe into our lungs 

 the atmospheric air, and swallow multitudes of these organisms. 

 But for it every wound would become putrid, every breath would 

 admit deadly germs to our blood. But when the vitality of the 

 part or of the body generally is lowered, the vital activity of the 

 tissue may fall below that of the bacteria, and their victory is 

 signalled by unwonted and often fatal changes. Morbid fluids 

 allowed to accumulate in the textures facilitate the growth of 

 bacteria, and give rise to various grades of " wound infection." 

 But if all accumulations be avoided, the bacteria brought into 

 relation with the living tissue can only irritate it, and cause gen- 

 eral fever and local suffering to the patient. They cannot pro- 

 pagate in live tissue as in lifeless fluids. As a rule, the injurious 

 effect of bacteria is in inverse proportion to the vital power of 

 the textures which they invade. This is seen in many cases 

 familiar to the physician and the surgeon. For instance, even 

 the bronchial mucous membrane may be unable to resist the 

 attacks of the atmospheric organisms. A person whose vital 

 powers are probably already low from repeated debauch, falling 

 asleep in the open air after excessive intemperance, and being 

 exposed to the reducing chill of night, may become so lowered 

 in vital activity, that putrefactive changes may begin in his lung 

 tissue. Indeed this is not an uncommon history in the beginning 

 of gangrene of the lung. 



We next come to forms of fungus, which set up a process very 

 like putrefaction, such as the yeast plant, Torula cerevisia, which 

 causes alcoholic fermentation in sugar solutions. In the torula 

 an external case containing protoplasm may readily be seen, and 



