TUBERCULOSIS IN ANIMALS. 267 



four organisms, always the one most commonly encoun- 

 tered. 



Ttjberculin. — The filtered products of growth from 

 old fluid cultures of the tubercle bacillus represent what 

 is known as tuberculin — a group of proteid substances 

 possessing most interesting properties. When injected 

 subcutaneously into healthy subjects tuberculin has no 

 effect, but when introduced into the body of the tuber- 

 culous person or animal a pronounced systemic reaction 

 results, consisting of sudden but temporary elevation of 

 temperature with, at the same time, the occurrence of 

 marked hypersemia round about the tuberculous focus, a 

 change histologically analogous to that seen in the 

 primary stages of acute inflammation. This zone of 

 hypersemia, with the coincident exudation and infiltra- 

 tion of cellular elements, probably aids in the isolation 

 or casting off of the tuberculous nodule, the inflamma- 

 tory zone forming, so to speak, a line of demarcation 

 between the diseased and healthy tissue. 



As a curative agent for the treatment of tuberculosis, 

 tuberculin has not merited the confidence that was at 

 first accorded to it. Its greatest field of usefulness is 

 now admitted to be as an aid to the diagnosis of obscure 

 cases, and more particularly those occurring in cattle, 

 where it has proven itself to be of inestimable value in 

 this particular application. 



Susceptibility of Animals to Tuberculosis. — 

 The animals which are known to be susceptible to the 

 tubercular processes are man, apes, cattle, horses, sheep, 

 guinea-pigs, pigeons, rabbits, cats, and field mice. 



White mice, dogs, and rats possess immunity against 

 the disease. 



We have reviewed the three common pathogenic 



