20 Cincinnati Society of Xaiiiyal History. 



and industn- on this subject. They liave achieved results of 

 not only high scientific interest, but of practical benefit. The 

 history of its development and present status has very much 

 of interest, and corresponds to that which has been known in 

 the historj' of many other scientific subjects. 



Progress has been slow, manj' obstructions and miscon- 

 structions have appeared, until now it has apparently solid 

 foundation, though it is evident much remains to be done. 



On this theory the germs are of vegetable origin. They 

 are exceedingly minute, being much smaller than the white 

 corpuscles of human blood — amazingly abundant in and out 

 of the human system — pervade air and water and the soil, and 

 grow with great rapidit}'. They have their anatomy, which is 

 of the simplest, such as that of protoplasm ; they have their 

 ph^'siolog}', chemistry and biological characteristics, that have 

 been elaborately studied and determined, and they have enor- 

 mous reproductive power. Their shapes are varied ; some 

 are rounded ; some rod-shaped and others are spiral. They 

 are called respectively micrococci, ])acilli and spirilli. The 

 rounded are the simjjlest forms of about .,--\»(i of an inch in 

 diameter. Some live without oxygen and others must have 

 it. The number of bacteria in the air is large : greater in 

 Summer than in Winter: greater at low levels than on 

 mountains, in the cities than out of them, in the dark than 

 in the ex])osed places. Their number diminishes after a rain. 

 They live at varying temperatures. 



There are two main divisions, so far as their modes of liv- 

 ing are concerned; one called Saprophytes, who live on dead 

 matter — animal or vegetable. They resolve it into carbonic 

 acid and ammonia, which serve as foods for the vegetable 

 world. They are not hurtful to the living human ])ody. 



The other division is called Parasites, and they obtain their 

 foods from human or vegetable matter. The products of their 

 life proces.ses are noxious to the living matter, and in IJiis 

 way this class l)ecomes the source of man\- diseases. 



Instances of benefits from bacteria are known in two 

 im])()rtant relations — they intervene in the i)rocess of diges- 

 tion and jHomote it. They form the "tertium ([uid " between 

 the structure of the stomach and its ultimate function diges- 

 tion. Tlie entire digestive tract is a productive " hal)itat" for 



