ZOOLOGICAL DISTRIBUTION OF DISEASE. 253 
Anthrax demonstrates in a remarkable manner why 
disease should have a zoological distribution, for slight 
physiological differences protect an animal against the 
action of the anthrax bacillus, the morbific agent of the 
ruinous splenic fever. This disease can easily be com- 
municated to the ox, sheep, rabbit, and guinea pig by 
injecting into the circulation a small quantity of blood 
taken from an animal which has died of splenic fever. 
. Such injections are rapidly fatal. On the other hand, it 
is difficult to inoculate the dog and pig, and fowls never 
acquire the disease. The cause of the immunity of 
fowls has been cleverly explained by Pasteur. It had 
been ascertained that the anthrax bacillus does not 
develop when subjected to a temperature of 44° Centi- 
grade. The body temperature of a fowl is about 41° C., 
Maiist that of the horse is 37°7 C., the dog and 
rabbit, 38°-39° C. On immersing the feet of a fowl in 
cold water at a temperature of 25° Cent. so as to 
reduce its body heat to 37° or 38°, and then injecting 
it with blood from a case of splenic fever, it was 
found at the end of twenty-four hours dead, with its 
blood filled with the bacteria of splenic fever. In another 
experiment a hen was inoculated and subjected to the 
cold-water treatment ; when the fever was at its height 
the hen was taken out of the water, wrapped carefully 
in cotton wool and placed in an oven at 35°C. In the 
course of a few hours it was restored to health. Hens 
killed after being experimented upon in this way exhibit 
no trace of the bacteria in their blood. 
Under ordinary conditions a frog cannot be killed by 
injection of anthrax cultures, but if, after inoculating a 
