Tuberculosis. 421 



less is true of animals. Pigs, with their large tonsils and fre- 

 quent infection through feeding, are probable examples of this,. 

 and when, in addition to the detention of the microbe in the cavi- 

 ties, there is an inflammation and debility of the tissues, the op- 

 portunity for infection is greatly enhanced. 



ENCREASED TRAFFIC IN (TUBERCULOUS) ANIMALS. DAIRY 

 EXTENSION. 



In common with all infectious diseases, tuberculosis owes 

 its great extensions to the sale and purchase of animals. Where 

 an indigenous race of cattle is raised and sold, without any addi- 

 tions from without, tuberculosis is usually rare and confined to 

 the one herd, or to those having intimate intercommunication 

 with it. Where, on the other hand, a large stock is kept up, as 

 in milk dairies, and few or no calves are raised, but the bulk of 

 the cows are sold off yearly and replaced by new purchases, 

 tuberculosis, sooner or later, finds admittance through the body 

 of an infected animal, and once implanted in the herd, it goes on 

 encreasing, without limitation, except by the sale or death of the 

 more seriously affected. The larger the herd and the greater the 

 number of yearly changes, the greater the number of opportuni- 

 ties for the introduction of an infecting animal. Where a great 

 part of the herd is turned off yearly, and the new animals are 

 drawn from any or every source indiscriminately, it would bor- 

 der on the miraculous if such a herd were to remain free from the 

 infection for any great length of time. The recent extension of 

 tuberculosis in herds has been coincident with the great develop- 

 ment of commerce in live stock, and the rapid transit on land and 

 water by steam. Before the days of the steam engine, animal 

 plagues advanced slowly and uncertainly, excepting in the pres- 

 ence of a great European war, which drew animals from all avail- 

 able sources, congregating them in large mutually infecting 

 droves in commisariat parks, and marching them in the wake of 

 the army for its daily supply. Inevitably every country invaded 

 was infected, and the plagues spread from the line of march in 

 all directions. The more deadly plagues, like rinderpest and 

 lung plague, were the first to appear, and in this lay a certain 

 measure of palliation of the evil, as the exposed and often weaker 

 animals were killed off, so that the slower contagion like that of 



