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 septicemia 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 microédrganism 
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 septiceemia, 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 
septiceemize heemorrhagicee) 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 in sufficient 
numbers. But, according to the estimates made, 225,009,000 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, 
