TUBERCULOSIS. 303 



water is absorbed with filter-paper, and then the slide is 

 heated over a Bnnsen burner until the section becomes 

 shining, when it receives a drop of xylol balsam and a 

 cover-glass. 



It is said that sections stained in this manner do not 

 fade as quickly as those stained by Ehrlich's method. 



The tubercle bacillus also stains well by Gram's method, 

 but as this is a general method by which many different 

 bacteria are colored, it is ill adapted for purposes of differ- 

 entiation, especially when the prosecution of the charac- 

 teristic methods is not more difficult. 



So far as is known, the tubercle bacillus is a purely 

 parasitic organism. It has never been found except in 

 the bodies and excretions of animals affected with tuber- 

 culosis, and in dusts of which these are component parts. 

 This purely parasitic nature greatly interferes with the 

 isolation of the organism, which cannot be grown upon 

 the ordinary culture-media. Koch first achieved its arti- 

 ficial cultivation by the use of blood-serum. When 

 planted upon this medium the bacilli are first apparent 

 to the naked eye in about two weeks, and occur in the 

 form of small dry, whitish flakes, not unlike fragments 

 of chalk. These slowly increase at the edges, and grad- 

 ually form scale-like masses of small size, which under 

 the microscope are seen to consist of tangled masses of 

 bacilli, many of which are in a condition of involution. 



The best method of obtaining a culture is to inoculate 

 a guinea-pig with tuberculous material, allow an artificial 

 tuberculosis to develop, kill the animal after a couple of 

 months, and make the cultures from the centre of one of 

 the tuberculous glands. 



Of course many technical difficulties must be over- 

 come. The sputum or other tuberculous material used 

 for inoculation may be injected beneath the skin by a 

 hypodermic syringe, or placed in a little subcutaneous 

 pocket made by snipping the skin of the thigh with scis- 

 sors and dissecting it loose so that the fragment is easily 

 introduced. The animal is allowed to live for a month 



