744 PROCEEDINGS OF SECTION H. 



substance called protoplasm, which is contained in an envelope 

 containing cellulose. The protoplasmic interior cannot have 

 the same chemical constitution in all bacilli, as these show 

 varied reactions to staining; and reag'ents. 



These microscopic organisms or microbes may be roughly 

 divided — so far as disease is concerned — into two main classes 

 — the pathogenic, or disease producing, and the non-pathogenic, 

 or comparatively harmless, which have no power of producing 

 an infective disease. They vary in shape, being round or 

 oval, when they are termed micro-cocci ; rod-shaped, more or 

 less elongated, when they are called bacteria, bacilli, or 

 vibriones ; spiral-shaped or spirilla ; and besides these forms, 

 there are wavy and filamentous varieties. 



Being excessively light in weight, and microscopically small 

 — varying from "OOOS mm. to "05 mm. in length or diameter, 

 they are easily carried about by the dust of the air, and in 

 consequence are present everywhere — in the air, in the soil, 

 and water. 



The large majority of the microbes that are found in the 

 air and soil are non-pathogenic ; but the pathogenic ones are 

 not absent, as their presence can be proved by the production 

 of pathological effects when introduced into the animal 

 system. 



Bacteria, like plants, form well defined and constant species, 

 and transmit their own particular likeness, although in some 

 cases they may show some variation owing to external 

 influences. Some observers believe that some non-pathogenic 

 bacilli can become pathogenic by cultivation, but this is not 

 generally regarded as correct. They each have their special 

 functions to perform, whether disease-producing or not, but 

 it is simply impossible for the harmless bacilli alone to produce 

 the same dire eifects that are only due to the pathogenic 

 forms. 



The micro-organisms of the infectious diseases have certain 

 selective powers, as they are apparently attracted to particular 

 organs, and find entrance each in its own particular way. 

 The diphtheritic bacillus, for instance, does not invade the 

 body generally, but attacks only the mucous membrane of 

 the throat, where it remains without going farther. 



In some diseases, such as hydrophobia, tetanus, and others, 

 the virus or contagium of the disease finds entrance only by 

 direct inoculation. In others, infection is conveyed through 

 the air; and in others again, the infective niatter is taken 

 into the system in the food. 



