310 PROTECTIVE INOCULATIONS. 
their cultures from the blood and from the various organs gave a 
negative result. 
Rabbits are not so susceptible, and may recover after the subcu- 
taneous inoculation of very small doses, but usually die in from four 
to twenty days when two to four cubic centimetres of a bouillon cul- 
ture have been introduced beneath the skin. In these animals, also, 
there is an extensive local cedema, an-enlargement of the neighboring 
lymphatic glands, and a fatty degeneration of the liver. Roux and 
Yersin have shown that in rabbits, when death does not ensue too 
quickly, paralysis of the posterior extremities frequently occurs, thus 
completing the experimental proof of the specific pathogenic power 
of pure cultures of this bacillus. 
Similar symptoms are produced in pigeons by the subeutaneous 
inoculation of 0.5 cubic centimetre or more, but they commonly re- 
cover when the quantity is reduced to 0.2 cubic centimetre (Roux and 
Yersin). 
The rat and the mouse have a remarkable immunity from the effects 
of this poison. Thus, according to Roux and Yersin, a dose of two 
cubic centimetres, which would kill in sixty hours a rabbit weighing 
three kilogrammes, is without effect upon a mouse which weighs only 
ten grammes. 
Old cultures are somewhat less virulent than fresh ones, but when 
replanted in a fresh culture medium they manifest their original viru- 
lence. Thus a culture upon blood serum which was five months old 
was found by Roux and Yersin to kill a guinea-pig in five days, but 
when replanted it killed a second animal of the same species in 
twenty-four hours. 
Evidently a microdrganism which destroys the life of a susceptible 
animal when injected beneath its skin in small quantity, and which 
nevertheless is only found in the vicinity of the point of inoculation, 
must owe its pathogenic power to the formation of some potent toxic 
substance, which, being absorbed, gives rise to toxemia and death. 
This inference in the case of the diphtheria bacillus is fully sustained 
by the results of experimental investigations. Roux and Yersin 
(1888) first demonstrated the pathogenic power of cultures which 
had been filtered through porous porcelain. Old cultures were found 
by these experimenters to contain more of the toxic substance than 
recent ones, and to cause the death of a guinea-pig in a dose of two 
cubic centimetres in less than twenty-four hours. The filtered cul- 
tures produced in these animals the same effects as those containing 
the bacilli—local cedema, hemorrhagic congestion of the organs, 
effusion into the pleural cavity. Somewhat larger doses were fatal 
