214 PHESIDETirT's ADDRESS. 



that the various forms may be referred roughly to four typical 

 genera, Micrococcus, a spherical cell ; Bacillus, a short rod-like 

 form ; Leptothrix, a long, slender thread ; Spirillum, a spirally 

 coiled thread. But in addition to these there are numerous forms 

 of intermediate or doubtful position. Perhaps the most generally 

 accepted primary division of the group is that indicated above, 

 dividing it into four corresponding families under the names 

 Spharo-hacteria, Micro-bacteria, Desmo-hacteria, Spiro-bacteria. 



That alcoholic fermentation is the result of the vital activity 

 of the "yeast-fungus" was clearly made out long ago, but it is 

 no less certain that changes of a somewhat similar nature are 

 brought about by the purely chemical action of certain " fer- 

 ments." Such is the action of diastase and ptyaline upon 

 starches, converting them into a soluble " grape-sugar," and of 

 pepsine upon the various proteids, turning them into soluble 

 peptones. So that, although the suspicion was strong that many 

 diseases might be produced by the vital action of microbes, there 

 was yet room for the belief that simple chemical changes were 

 the more common cause of morbid manifestations. Modern 

 researches, however, clearly show that the organic chemical 

 poisons, — tox-albumens and the rest — which are so hurtful to 

 the animal economy, are themselves produced for the most part 

 by the growth of microbes. One of the simplest and most in- 

 telligible forms of bacterial disease is the ordinary decay or caiies 

 of teeth. It has long been known that various kinds of non- 

 pathogenic Bacteria may be found in the mouth, teeth and gums. 

 Some of these produce lactic acid from the sugars of the food, 

 and this acid, acting upon the teeth, softens them and allows of 

 the penetration of certain thread-like forms [LeptotJirix) which 

 by theii' growth rapidly disintegrate the tooth. 



Still more recent researches have proved to demonstration that 

 many diseases — such as have been called *' zymotic," as well as 

 others — are caused by the attacks of special microbes, each 

 disease having one peculiar to itself. These microbes have, in 

 many cases, been separated, cultivated, and shown to be capable 

 of producing their special diseases when introduced into the 

 animal economy. Among the diseases whicli have been shown 



