MODES OF ACTION. 227 



functions by appropriating for their own use material required for 

 the nutrition of the tissues ; and at the same time toxic substances 

 are formed which play an important part in the production of the 

 morbid phenomena, which in this class of diseases very commonly 

 lead to a fatal result. The pathogenic bacteria which invade the 

 blood may also, in certain cases, give rise to local necrosis and dis- 

 turbance of function in various organs in a mechanical way by 

 blocking up the capillaries. 



The invasion of the blood which occurs in anthrax and in vari- 

 ous forms of septicaemia in the lower animals, induced by subcuta- 

 neous inoculation with pure cultures of certain pathogenic bacteria, 

 does not generally immediately follow the inoculation. Usually a 

 considerable local development first occurs, which gives rise to more 

 or less inflammation of the invaded tissues, and very commonly to 

 an effusion of bloody serum in which the pathogenic microorganism 

 is found in great numbers. Even in susceptible animals the blood 

 seems to offer a certain resistance to invasion, which is overcome 

 after a time by the vast number of the parasitic host located in the 

 vicinity of the point of inoculation, aided probably by the toxic sub- 

 stances developed as a result of their vital activity. 



The experiments of Cheyne (1886) seem to show that in the case 

 of very pathogenic species, like the anthrax bacillus or Koch's bacil- 

 lus of mouse septicaemia, a single bacillus introduced subcutaneously 

 may produce a fatal result in the most susceptible animals, while 

 greater numbers are required in those which are less susceptible. 

 Thus a guinea-pig succumbed to general infection after being inocu- 

 lated subcutaneously with anthrax blood diluted to such an extent 

 that, by estimation, only one bacillus was present in the fluid in- 

 jected ; and a similar result in mice was obtained with Bacillus 

 murisepticus. In the case of the microbe of fowl cholera (Bacillus 

 septicaemias haemorrhagicae) Cheyne found that for rabbits the fatal 

 dose is 300,000 or more, that from 10,000 to 300,000 cause a local 

 abscess, and that less than 10,000 produce no appreciable effect. 

 The common saprophyte Proteus vulgaris was found to be patho- 

 genic for rabbits when injected into the dorsal muscles hi sufficient 

 numbers. But, according to the estimates made, 225,000,000 were 

 required to cause death, while with doses of from 9,000,000 to 112,- 

 000,000 a local abscess was produced, and less than 9,000,000 gave 

 an entirely negative result. 



Secondary infections occurring in the course of specific infec- 

 tious diseases are of common occurrence. Thus a pneumonia may 

 be developed in the course of an attack of measles or of typhoid 

 fever ; or infection by the common pus cocci in the course of scarlet 

 fever, typhoid fever, mumps, etc., may give rise to local abscesses, 



