MORPHOLOGY AND PHYSIOLOGY OF BACTERIA. 29 



health we inhale with impunity. We know that a microbe may only 

 cause a local lesion in one animal, but death in another. It is still 

 more striking that the same micro-organism, as is the case with 

 anthrax, may have no effect whatever upon certain species of 

 animals, though it is deadly to others. Again, an animal naturally 

 susceptible to the effect of a pathogenic organism may be rendered 

 proof against it. These matters will be discussed in a future, 

 chapter. 



Distribution of Bacteria. 



Bacteria, are commonly described as ubiquitous. They are ever 

 present in the air, though not in such exaggerated numbers as is 

 commonly supposed. In nutrient media exposed to the air one is often 

 astonished at times at the comparatively few bacteria which develop 

 in comparison to the amount of floating matter, such as mineral 

 particles, scales, spores of fungi and debris known to be present. 

 In water they are also present in considerable numbers, though of 

 course varying according to the character of the water. Wherever 

 there is putrefaction, they are present in vast numbers. In the soil, 

 in sewage, in the intestines and, in uncleanly persons especially, 

 on the skin and between the teeth, various species may always be 

 found, but in the healthy blood and healthy tissues bacteria are 

 never present. In a previous chapter the method of examining the 

 blood of living persons has been described, and there is, by this 

 means, ample opportunity for satisfying oneself that bacteria are 

 never to be found in the blood in health. The organs removed from a 

 perfectly healthy animal, with the necessary precautions, and placed 

 in steriUsed media, can be kept indefinitely without undergoing 

 putrefaction, or giving any development of bacteria. This has been 

 estabHshed by many observers, notably Cheyne and Hauser ; and 

 the results of former observers to the contrary must be attributed 

 to imperfect methods admitting of accidental contamination. 



