TUBERCULOSIS IN CATTLE. 17 
tion of the time, bovine tuberculosis is much more prevalent, 
and the amount of tuberculosis is roughly parallel to the ex- 
tent of indoor life. Where the cattle are kept housed all the 
time, the amount of the disease is very large. ‘There are two 
or three factors which probably explain this fact. In the first 
place, animals living indoors do not have as much air, and 
the activity of their lungs is impaired thereby. They are 
therefore more subject to an attack of the disease than ani- 
mals living outdoors, where air has more easy access to the 
lungs and where vigorous exercise in the open fields keeps 
the lungs in a more active condition. Secondly, a very 
prominent factor is, doubtless, the fact that the tubercle bacil- 
lus has a very much more easy chance of access to animals 
in the stall than out of doors. Inside of the barn the cattle 
come in close contact with each other, and there is every 
possible means by which the germ can pass from one animal 
to the other. Out of doors there is not this close contact. 
Thirdly, it is a well known fact that the sunlight quickly de- 
stroys tubercle bacillus together with all other bacteria, and, 
therefore, if the animals live in the fields the tuberculosis bacilli — 
which are excreted from them in any way are very rapidly killed 
by the rays of sunlight, so that after a comparatively few hours 
they are harmless. In the dark stable, however, they may re- 
main alive and active for months. These three factors together 
very largely explain the greater prevalence of the disease in 
housed animals. It must not, however, be understood that 
tuberculosis is absent from animals that live out in the fields. 
It occurs occasionally in animals who never enter the barn, but 
the amount of disease among animals living in the free air is 
very decidedly less than in those living indoors. 
We find, furthermore, that the amount of tuberculosis 
varies with the breed of the animal. There is no general agree- 
ment as to what breeds of animals suffer most. Some claim 
that Jerseys, Shorthorns, and Ayrshires are especially sus- 
ceptible, but it is doubtful whether this is true. In general, 
the animals of high pedigree stock are more subject to the 
disease than ordinary cattle. The reason for this is probably 
not especially connected with the breed, but rather with the 
conditions under which they are kept. These valuable ani- 
mals are usually great milkers, and their general vitality is 
somewhat lowered by the strain upon the animal to produce 
milk. This lowering of the vitality by a great production of 
milk renders the animal somewhat more liable to attack than 
