262 SUSCEPTIBILITY AND IMMUNITY. 
the result appears to depend directly upon the destruction of the 
pathogenic microérganism in the body of the immune animal. In 
guinea-pigs which have an acquired immunity against Vibrio Metsch- 
nikovi the blood serum has been proved to possess decided germicidal 
power for this ‘‘ vibrio,” whereas it multiplies readily in the blood 
serum of non-immune guinea-pigs (Behring and Nissen). 
There is experimental evidence that animals may acquire an arti- 
ficial immunity against the toxic action of certain toxalbumins from 
other sources than bacterial cultures. Thus Sewell (1887) has shown 
that a certain degree of tolerance to the action of rattlesnake venom 
may be established by inoculating susceptible animals with small 
doses of the “‘hemialbumose” to which it owes its toxic potency. 
These results have been confirmed by the more recent experiments of 
Calmette (1894) and of Fraser (1895). In his paper detailing the 
results of his experiments the first-named author says: 
‘‘Animals may be immunized against the venom of serpents either by 
means of repeated injections of doses at first feeble and progressively stronger, 
or by means of successive injections of venom mixed with certain chemical 
substances, among which I mention especially chloride of gold and the hypo- 
chlorites of lime or of soda. 
“The serum of animals thus treated is at the same time preventive, anti- 
toxic, and therapeutic, exactly as is that of animals immunized against 
diphtheria or tetanus. 
“Tf we inoculate a certain number of rabbits, under the skin of the 
thigh, with the same dose, one milligramme of cobra venom for example, 
and if we treat all of these animals with the exception of some for control, 
by subcutaneous or intraperitoneal injections of the serum of rabbits im- 
munized against four milligrammes of the same venom, all of the control 
animals not treated will die within three or four hours, while all of the 
animals will recover which receive five cubic centimetres of the therapeutic 
serum within an hour after receiving the venom.” 
In this connection we may remark that there is some evidence to 
show that persons who are repeatedly stung by certain poisonous in- 
sects—-mosquitoes, bees—acquire a greater or less degree of immu- 
nity from the distressing local effects of their stings. 
Ehrlich, of Berlin, in 1891, reported his success in establishing 
immunity in guinea-pigs against two toxalbumins of vegetable 
origin: one—ricin—from the castor-oil bean (Ricinus communis), 
the other—abrin—from the jequirity bean. The toxic potency 
of ricin is somewhat greater than that of abrin, and it is esti- 
mated by Ehrlich that one gramme of this substance would suffice 
to kill one and a half millions of guinea-pigs. When injected be- 
neath the skin, in dilute solution, it produces intense local inflamma- 
tion, resulting in necrosis of the tissues. Mice are less susceptible 
than guinea-pigs and are more easily made immune. This is most 
readily effected by giving them small and gradually increasing doses 
with their food. As a result of this treatment the animal resists 
