METHODS OF DISTRIBUTION. 267 
to his boots will be sure to be knocked off when dry, and will thus 
be carried everywhere that the farmer goes. They will be certain 
to be dislodged near a healthy cow and may become mixed with 
her food which is commonly thrown on the floor in front of her; or the 
particles may become dry and be distributed through the barn as 
dust. In short it is inevitable that the bacilli voided with the 
excrement will in time come in contact with every healthy animal 
kept in the same barn. 
Once distributed from infected animals, the bacilli may find 
entrance into the healthy animals, by a variety of channels. Some 
find entrance to the lungs, either by the dust particles or by the 
bacilli-laden moisture drops from coughing animals, which are 
breathed by healthy animals. The bacilli which find their way 
into the watering trough will be swallowed, and the same will be 
true of those which the animal takes into its mouth by licking its 
infected neighbor. These two means of entrance are doubtless 
responsible for most cases of bovine tuberculosis, and it is very 
easy to understand how a single diseased animal in a barn may, 
in time, infect most of the herd. 
Abundance of Bovine Tuberculosis. Tuberculosis is widely 
distributed among cattle, although it is by no means universally 
found in countries where cattle are kept. It is said not to occur 
in Africa, and until recently it has been absent from China and 
Japan, having lately been introduced with imported cattle. In 
the western part of the United States, among the cattle living out 
of doors most of the time, it is rare or absent. In general it is most 
abundant in localities where the cattle are housed for a considerable 
part of the year. It is consequently most abundant in northern 
countries, and appears to be most widely distributed in northern 
Europe. 
It is practically impossible to state the percentage of cattle 
suffering from tuberculosis. Among the animals examined in the, 
slaughter houses of Denmark it has sometimes appeared that more 
than half of the cows are tuberculous. From these high figures 
the percentage has ranged down to 10 per cent, or even lower in 
some places, and, in fact, is so variable that no general averages 
