Its relation to Consumption in Man. 
313 
although the disease has been produced experimentally by inocu- 
lation, inhalation, and ingestion, many people do not believe in 
its contagious nature — at least not in the ordinary sense. In the 
case of small-pox, scarlet fever, and other transmissible diseases, 
with a short and definite period of incubation, the date of the 
exposure to the risk of infection is often known and remembered ; 
but in the case of a disease like tubercle, in which the earlier 
symptoms are not observed, or are ill-defined, and the progress is 
very slow, the time of exposure to infection cannot be so readily 
fixed. Many who admit that a man may catch small-pox, or 
scarlet fever, or an animal may take foot-and-mouth disease, from 
having been in contact with patients suffering from those affec- 
tions, would laugh at the idea of either men or animals catching 
consumption. 
Many cases are, however, recorded in the human subject, of 
consumptive patients infecting other people with whom they 
were in the habit of daily associating. It cannot be denied that 
such communication is possible; but experience indicates that 
it does not happen very often. In consumption-hospitals the 
nurses are daily and constantly exposed to the risk of infection 
in this way : the tubercle-bacillus has been found abundantly in 
the air of the hospital wards ; but still, it is the exception to find 
the nurses fall victims to this disease. It must be admitted, there- 
fore, that the infective power of consumption in man is very low, 
and unless the person exposed is predisposed either by heredity, 
unhealthy surroundings, or other debilitating influences, the 
infection is resisted, because the organism is not introduced into 
a suitable soil for its growth and reproduction. Were it other- 
wise, consumption would no doubt have steadily increased, 
whereas it has of late years considerably decreased. 
In a similar manner the infective power of tubercle in 
animals is very low. Practical men consider that the affection 
is not readily communicable from one animal to another, even 
when they are in close association, as in the case of cows in town 
dairies. It may happen that one or two animals in a herd may 
be affected with tubercle and gradually waste until the owner 
either kills them and buries the carcass, or sells the animal to 
the jobber for about the value of the hide. Yet no apprehension 
is felt with regard to the rest of the herd, such as there would 
be had an animal affected with pleuro-pneumonia been brought 
into it. It may be argued that the stock-owner's opinion as to 
the infectious nature of a disease is not of much value ; but it 
must be admitted that any man of long experience can give the 
results of his own observations, and the evidence of practical men 
generally is to the effect that tubercle does not spread like other 
