8 Bulletin 150. 



herd, in the natural course of events it must become better and 

 better adapted to survival in that particular animal and breed, 

 and hence increasingly dangerous to all of its members. This 

 is one reason why tuberculosis is so liable to become intensified 

 in special herds of thoroughbred stock, and why common cattle 

 with a varied ancestrj' will sometimes seem to offer a longer resist- 

 ance to the affection. It may also explain the fact that with 

 ample exposure the disease does not always pass from men to 

 cattle and from cattle to man. 



Yet it would be folly to argue from such data that the disease, 

 when present in an occult form in a herd, may be safely ignored, 

 and that the products of such herd may be safely consumed by 

 man. The ver}^ adaptability of the tubercle bacillus sufficiently 

 contradicts this conclusion. The mere continuous presence of 

 the bacillus in a given system, human or brute, is the means of 

 securing a better and still better adaptation to that form of life, 

 and a greater and still greater measure of potency, so that when 

 the health of the host or exposed animal is in any way reduced 

 it may at once become deadly and far reaching in its evil effects. 



CHANNELS OF INFECTION. 



Among the channels of infection the following may be noted : 

 I. Inhalation by the breath. This is perhaps the most com- 

 mon method of infection and is usualh' followed by tuberculosis 

 of the throat, lungs, and hmiphatic glands of the chest. Expec- 

 torations and other infecting discharges are dried up and raised 

 in dust so that they can be easil}' inhaled. Cases of this kind 

 have been observed in buildings in which a victim of advanced 

 tuberculosis was employed. The other employes fell victims, 

 one after another, to the infection. The}^ are quite common in 

 infected barns, in which the virulent dust carried in the air is 

 inhaled by a number of animals. Experimentally it has been 

 shown by mixing virulent matters in liquids, atomizing them 

 and causing animals to inhale the spra3\ In the hands of Vill- 

 emin, Koch, Thaon and Tappeiner this almost infallibly produced 

 tuberculosis of the lungs. In man too, many infections and 

 reinfections have been traced to the dust from the soiled handker- 

 chiefs. On the other hand it must be distinctlv understood that 



