DIFFERENTIAI, DIAGNOSIS 151 



cular lesions open into the respiratory tract the specific bac- 

 teria can ahnost always be found in the expectoration. This 

 is especially true in men and Ravenel has shown that it is often 

 true in cattle. In old and in the very recent tuberculous 

 lesions, it is not so easy to detect these organisms microscopic- 

 ally. When there is doubt the animal inoculation gives quite 

 prompt results. 



§ 116. Animal inoculation for purposes of diagnosis. 

 Guinea pigs are preferable, although rabbits may be used. 

 With tuberculous tissue either of two methods may be em- 

 ployed, (i) A small piece (about the size of a pea or bean) of 

 the tissue may be inserted under the skin by first making an 

 incision with a .sharp scalpel through the skin and superficial 

 fascia, and then with a pair of fine forceps insert the bit of 

 tissue well under the skin and close the opening with one or 

 more sutures. (2) The tissue may be crushed in a mortar and 

 thoroughly mixed with a few cubic centimeters of sterile w^ater 

 or bouillon and then injected with a hypodermic syringe. The 

 needle should be of large calibre. If it is suspected milk it 

 may be injected into the abdominal cavity. If the material is 

 tuberculous and contains living tubercle bacteria, the death of 

 the animal follows in from three weeks to four months. Usually 

 the lymphatic glands in the groin and axilla are enlarged and 

 often caseous. If a guinea pig is u.scd, the liver, spleen, lungs 

 and kidneys are liable to be affected, in the order named ; if a 

 rabbit, the lungs are often the first of the viscera to be 

 attacked. 



§ 117. Tuberculin test. The tuberculin test is the best 

 practically and in a large majority of tuberculous cases among 

 animals and in man the only means of positively detecting the 

 di.sease in the living individual. 



Tuberculin. Tuberculin is the concentrated liquid, usually 

 glycerinated bouillon, on which tubercle bacteria have grown 

 until the products resulting from their multiplication have 

 become imparted to the medium in sufficient quantity to inhibit 

 their further development. It is not definitely determined just 

 what these products are or just how they are elaborated. 

 Briefly stated, the preparation of tuberculin consists in the 

 following procedures, viz : 



