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2- /. IVACCiNE* AND .SEBUM THERAPY 



more extensive* areas 'of the animal tissues, ^nile 

 others may remain localised at the point of invasion 

 and exert their harmful action chiefly by local growth 

 and the elaboration of specific poisons. 



The inciting or inhibiting factors which permit or 

 prohibit an infection are dependent in part upon the 

 nature of the invading germ and in part upon the 

 defensive mechanism of the subject attacked. Bacteria 

 are roughly divided into two classes — saprophytes and 

 parasites. The saprophytes are those bacteria which 

 thrive best on dead organic matter, and fulfil the 

 enormous important function in nature of reducing 

 by their physiological activities the excreta and dead 

 bodies of more highly organised forms into those 

 simple chemical substances which may again be 

 utilised by the plants in their constructive processes. 

 Parasites, on the other hand, find the most favourable 

 conditions for their development upon the living 

 bodies of higher forms. 



While a strict separation of the two divisions cannot 

 be made, numerous species forming transitions 

 between the two, it may be said that the latter class 

 comprise most of the so-called pathogenic bacteria. 

 Strict sapropliytes may cause disease, but only in 

 cases where other factors have brought about the death 

 of some part of the tissues, and the bacteria invade 

 the necrotic areas and break down the proteids into 

 poisonous chemical substances such as ptomains, or, 

 through their own destruction, give rise to the libera- 

 tion of the toxic constituents of their bodies. It is 



