GENERAL MORPHOLOGY AND PHYSIOLOGY. 149 



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 oppor- 

 tunity 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, into sterilised media can be 

 kept indefinitely without undergoing putrefaction, 

 or giving any development of bacteria. This has 

 been established 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 con- 

 tamination. 



