86 THE DISEASES OF DOMESTIC ANIMALS. 



organisms already in existence, they represent nothing more than 

 different stages or forms of development of known species. This 

 latter hypothesis is known as polymorphism. 



The Dissemination of Bacteria ra Different Media — Air and 



Water. 



The experiments of Pasteur, Tyndall, and others, have clearly 

 demonstrated the presence of vegetable germs in the air which has 

 been allowed to pass through apertures into vessels prepared for the 

 purpose. These germs generally have the micrococcus or microbe 

 form, and are present in lesser numbers in winter than in summer 

 and fall, which fact finds its explanation in the greater degree of 

 vegetable decomposition and telluric evaporation which takes place 

 during the warm months in comparison with the cold. 



The experiments of Cohn and others have demonstrated that the 

 atmosphere contains very few adult bacteria ; in fact, they are rarely 

 found therein in a complete state, but rather as bright, refracting 

 points, very difficult if not impossible to distinguish from one another. 

 These points probably represent the latent or permanent spore-con- 

 dition of bacterium life, capable of generating into true bacteria 

 under favorable conditions. These spores may form the point from 

 which epidemics take their origin, and in this condition are capable 

 of wide dispersion. 



Nutrition and Respiration of Bacteria. 



As bacteria have some of the essential characteristics of organ- 

 ized beings — a cell-membrane and protoplasmic contents — they must 

 naturally receive nourishment and respire in the same way as all 

 colorless vegetables and inferior animals which have no special 

 apparatus for such purposes ; that is, by endosmotic absorption. 



It matters not in what medium they may be met with, they re- 

 quire — in order to live — water, nitrogen, carbon, and oxygen, as well 

 as certain salts from the mineral world, which enter in very minute 

 quantities into their organism. 



Water is indispensable to the activity and development of bac- 

 teria. Desiccation, drying out, completely arrests the movement of 

 those that are mobile, and the functional activity of all bacteria. 

 Desiccation does not, however, kill them, unless it is too prolonged, 

 as is proved by the activity retained by the various kinds of virus 

 used for inoculation. In the condition of permanent spores they 

 retain their vitality a long while. It is a surprising fact that the 

 great chemical difference existing between salt and fresh water 



