282 AGRICULTURAL BACTERIOLOGY 



which can be easily determined by thermometer readings 

 before and after the subcutaneous injection of the tuber- 

 culin. 



In making tuberculin, the tubercle bacillus is grown in 

 beef broth to which glycerine has been added. After the 

 maximum growth has been obtained the cultures are heated 

 to kill the bacilli and extract all of their soluble cell prod- 

 ucts. This extract is concentrated to a definite volume, and 

 then filtered through porcelain filters to remove all the dead 

 organisms. It is, of course, impossible for the tuberculin 

 to produce tuberculosis, as has often been stated by those 

 ignorant of its nature. When this material is brought in 

 contact with the tissues of a healthy animal, no effect is to 

 be noted, while in the case of a tubercular animal, an effect 

 varying with the method of application is produced, because 

 the tissues of the diseased animal are supersensitive to the 

 tuberculin. 



There are at present three methods of applying tubercu- 

 lin : to the eye, when it is known as the ophthalmic test ; 

 into the skin, when it is called the intradermal test; anji 

 beneath the skin, as it was originally used, when it is re- 

 ferred to as the subcutaneous or thermal test. In the 

 former a drop of tuberculin is placed on the eyeball. A 

 more or less marked inflammation results in a few hours in 

 the case of a tubercular animal. If the tuberculin is intro- 

 duced between the layers of the skin, a swelling results 

 which is more extensive and persists for a longer period 

 in a tubercular than in a healthy animal. The subcutane- 

 ous test is the more reliable and used far more extensively 

 than the others, although it takes a considerably longer time 

 to produce the desired reaction. In addition to the tem- 

 porary fever produced in a reacting animal by the sub- 

 cutaneous injection of tuberculin, a constitutional reaction 

 is frequently noted, as is indicated by loss of appetite, in- 



