PROTECTIVE INOCULATIONS. 355 
trate from a virulent culture with blood serum from an immune animal 
and allowing it to stand for twenty-four hours; a dose three hundred 
times greater than would have sufficed to kill a mouse proved to be 
without effect after such admixtures with blood serum; as before 
stated, the blood serum of animals which are not immune has no 
effect upon the poison. The duration of immunity induced in this 
way was from forty to fifty days. Blood serum from an immune rab- 
bit, preserved in a cool, dark room, retains its power of neutralizing 
the tetanus poison for about a week, after which time it gradually 
loses it. Having found that chickens have a natural immunity against 
tetanus, Kitasato made experiments to ascertain whether their blood 
serum would also neutralize the tetanus poison; the result was nega- 
tive. 
That the tetanus poison is present in the blood of individuals who 
die from tetanus has been proved by Kitasato by injecting a small 
quantity (0.2 to 0.3 cubic centimetre) of blood from the heart of a 
fresh cadaver into mice; the animals develop typical tetanic symp- 
toms and die in from twenty hours to three days. 
Tizzoni and Cattani have (1891) reported results similar to those 
obtain by Kitasato. By repeated inoculations with gradually in- 
creasing doses of the tetanus poison they succeeded in making a dog 
and two pigeons immune, and found that blood serum from this im- 
mune dog, in very small amount, completely destroyed the toxic 
power of a filtrate from cultures of the tetanus bacillus—one to two 
drops of serum neutralized 0.5 cubic centimetre of filtrate after fifteen 
to twenty minutes’ contact. They also ascertained that small amounts 
of blood serum from this immune dog injected into other dogs or 
white mice produced immunity in these animals; but they were not 
able to produce immunity in guinea-pigs or rabbits by the same 
method. 
In a later communication (May, 1891) Tizzoni and Cattani give 
an account of their experiments made with a view to determining 
the nature of the substance in the blood serum of an immune animal 
which has the power of destroying the toxalbumin of tetanus—“tet- 
anus antitoxin.” They found, in the first place, that this antitoxin 
in blood serum is destroyed in half an hour by a temperature of 68° 
C.; further, that it does not pass through a dialyzing membrane; that 
it ig destroyed by acids and alkalies. As a result of their researches 
they conclude that it is an albuminous substance having the nature of 
an enzyme. 
Vaillard has succeeded in producing immunity in rabbits by re- 
peated injections into the circulation of filtered cultures—in all twenty 
