176 RELATIONS OF BACTERIA TO DISEASE 



of anthrax in white mice. The healthy peritoneum of a rabbit 

 can resist and destroy a considerable number of pyogenic 

 micrococci without any serious result, but if a larger dose be 

 introduced, a fatal peritonitis may follow. There is, therefore, 

 for a particular animal, a minimum lethal dose which can be 

 determined by experiment only ; a dose, moreover, which is 

 modified by various circumstances difficult to control. 



The path of infection may alter the result, serious effects often 

 following entrance into the blood stream. Staphylococci in- 

 jected subcutaneously in a rabbit may produce only a local 

 abscess, whilst on intravenous injection multiple abscesses in 

 certain organs may result and death may follow. Local inflam- 

 matory reaction with subsequent destruction of the organisms 

 may be restricted to the site of infection or may occur also in 

 the related lymphatic glands. The latter therefore act as a 

 second barrier of defence, or as a filtering mechanism which aids 

 in protecting against blood infection. This is well illustrated in 

 the case of " poisoned wounds." In some other cases, however, 

 the organisms are very rapidly destroyed in the blood stream, 

 and Klemperer has found that, in the dog, subcutaneous injection 

 of the pneumococcus produces death more readily than intra- 

 venous injection. 



In the case of syphilis, inoculation of monkeys is more 

 successful by scarification than by any other means. 



2. The Subject of Infection. — Amongst healthy individuals sus- 

 ceptibility to a particular microbe may vary according to (a) 

 species, (6) race and individual peculiarities, (c) age. Different 

 species of the lower animals show the widest variation in this 

 respect, some being extremely susceptible, others highly resistant. 

 Then there are diseases, such as leprosy, syphilis, etc., which under 

 natural conditions are peculiar to the human subject and can only 

 be transmitted to a few of the animals. And further, there are 

 others, such as cholera and typhoid, the typical lesions of which 

 cannot be experimentally reproduced in animals, or appear only 

 imperfectly, although pathogenic effects follow inoculation with 

 the organisms. In the case of the human subject, differences in 

 susceptibility to a certain disease are found amongst different 

 races, and also amongst individuals of the same race, as is well 

 seen in the case of tubercle and other diseases. Age also plays 

 an important part, young subjects being more liable to certain 

 diseases, e.g., to diphtheria. Further, at different periods of life 

 certain parts of the body are more susceptible, for example, in 

 early life, the bones and joints to tubercular and acute suppura- 

 tive affections. 



