266 TUBERCULOSIS. 
METHODS OF DISTRIBUTION. 
Tuberculosis is contagious. By this is meant that the relation 
of the bacillus to the animal is such that there is an easy means 
of communication between one animal and another under the 
ordinary conditions of life. The knowledge of this fact in regard 
to human consumption has been of great value, since it has been 
followed by a steady decline in the amount of the disease. Such 
knowledge has not yet reduced the amount of bovine tuberculosis. 
We can easily understand the methods of contagion when we 
remember that the bacilli are discharged from any of the open 
tubercles. If the disease is located only in an internal lymphatic 
gland it may not result in breaking down the gland, and there may 
be no discharge. Under this condition there is no contagion from 
one animal to another. But if it be located in the lungs the bacilli 
will be discharged into the air passages and pass through the trachea 
into the mouth. They will then infect all the discharges from the 
mouth and nose. It is true that the cow does not expectorate, but 
by putting her nose in the drinking trough she will be sure to con- 
taminate the drinking-water, and when she licks another animal, as 
she will be sure to do if she stands near others, she will leave some of 
the bacilli clinging to the second individual, ready to begin their mis- 
chief if they chance to get carried to a susceptible part, as they are 
very likely to do by being swallowed. Moreover, since the cow does 
not expectorate, she does swallow the secretions from her mouth. 
The tubercle bacilli will thus be carried to the stomach, and through 
the intestine, from whence they will be voided with the excrement. 
If the disease is located in the intestine the bacilli will be sure to be 
discharged with the excrement. In these ways the excrement of 
tuberculous cattle is sure to be impregnated with the bacilli. Now 
the conditions of the ordinary cow stall, even in the best cow barn, 
are such as to make it almost inevitable that the infectious material 
will soon be distributed through the whole barn. The excrement 
may be carried over the floor, perhaps, for some distance to the 
opening used for its exit, and the farmer's boots will always collect 
more or less and carry it through the barn. The particles adhering 
