618 THE POPULAR SCIENCE MONTHLY. 



bacillus as obtained from the spleen of typhoid cadavers varies 

 somewhat in different cases; that very similar bacilli may be 

 obtained from waters contaminated by sewage, etc., which differ 

 from the typhoid bacillus in certain characters of growth, and 

 yet resemble it so closely that it is still uncertain whether they are 

 to be considered distinct species or only varieties of the typhoid 

 bacillus ; that the character which was at first supposed to dis- 

 tinguish the typhoid bacillus from all others viz., its invisible 

 growth upon potato has proved to be unreliable, inasmuch as 

 certain other bacteria have been shown to have a like invisible 

 growth, and under certain circumstances the typhoid bacillus may 

 form a visible growth on potato. However, in spite of these 

 difficulties in differentiating the typhoid bacillus from nearly 

 allied bacteria found in water or in the dejecta of man and the 

 lower animals, there is good reason to believe that the bacillus of 

 Eberth, of Koch, and of Gaffky is the veritable etiological agent 

 in the widely spread endemic and sometimes epidemic disease 

 known as typhoid fever. But in admitting this we must admit 

 that the bacillus itself is widely distributed, and that an attack of 

 typhoid fever does not necessarily follow its introduction into the 

 alimentary canal of man by means of contaminated water or milk. 

 Other secondary causes, no doubt, often determine the question of 

 infection. Among these we may mention individual suscepti- 

 bility ; exposure to agencies which reduce the vital resisting 

 power, such as sewer-gas poisoning; the quantity and the viru- 

 lence of the typhoid germs ingested; the state of the digestive 

 function, etc. 



In 1880 the present writer discovered the important pathogenic 

 micrococcus which is now generally recognized as the usual cause 

 of croupous pneumonia. This I now call Micrococcus pneumonic? 

 crouposce: The German bacteriologists usually speak of it as the 

 Diplococcus pneumonicB. I first discovered this micrococcus in 

 the blood of rabbits inoculated with a few drops of my own saliva ; 

 and subsequent researches have shown that it is found in the 

 saliva of healthy individuals in various parts of the world. This 

 fact may at first view appear to be opposed to the statement that 

 it is the usual cause of croupous pneumonia, especially as I have 

 never myself suffered from this disease, although for several years 

 I frequently demonstrated the presence of this micrococcus in my 

 salivary secretions. But, as in the case of the typhoid bacillus 

 and several other widely distributed bacteria, while accepting, 

 upon experimental evidence, the etiological relation of the specific 

 micro-organism, we are also obliged to admit the essential rela- 

 tion of predisposing or exciting causes in the development of an 

 attack of the disease. In this connection I quote from a paper of 

 my own, published in 1885 : 



