CAUSING CHRONIC INFECTIONS. 95 



cellular or soluble toxin, to which is attributed the 

 fever, headache, loss of appetite, and so on, and an 

 endotoxin which causes the irritation of the tissues 

 leading to the formation of the tubercle. The ab- 

 sorption of these toxins causes the formation of anti- 

 bodies, but not in sufficient amount to cause immunity. 

 The toxins of the tubercle bacilli may be obtained from 

 cultures, and are used under the name of tuberculin 

 in the diagnosis and treatment of the disease. The Tuber . 

 tuberculin reaction used in the diagnosis is based upon 

 an observation made by Professor Koch, that animals 

 having tuberculosis were very sensitive to the poison, 

 and when injected with even a small amount of tuber- 

 culin developed fever, headache, nausea, vomiting, and 

 general malaise, while the diseased tissues became 

 temporarily more inflamed. Healthy animals were 

 unaffected. This method has been employed among 

 tuberculous patients, using from i to 10 milligrams of 

 the tuberculin. Simpler methods have more recently 

 been used, such as the von Pirquet test, in which the 

 tuberculin is introduced into the superficial layers of 

 the skin with a scarifier, and the Moro test, in which 

 the tuberculin is rubbed in, in the form of an ointment. 

 In the first method a positive reaction is manifested by 

 fever, headache, and so on, as described above, but in 

 the cutaneous tests there is only a local redness about 

 the point of inoculation. A positive test means that 

 tuberculosis is present in the body, but it do^s not tell 

 us where or whether it is active or not. In children, 

 a positive reaction usually means active disease. 



