SPECIFIC REACTIONS OF TUBERCLE BACILLI 289 



less risk of infection by means of the flesh of tubercular animals, 

 for, in the first place, tuberculosis of the muscles of oxen being 

 very rare, there is little chance of the bacilli being present in 

 the flesh unless the surface has been smeared with the juice of 

 the tubercular organs, as in the process of cutting up the parts ; 

 and, in the second place, even when, present they will be de- 

 stroyed if the meat is thoroughly cooked. 



We may state, therefore, that the two great modes of infection 

 are by inhalation, and by ingestion, of tubercle bacilli. In 

 the former, the tubercle bacilli will in most cases be derived 

 from the human subject ; in the latter, probably from tubercular 

 cows, though inhaled tubercle bacilli may also be swallowed and 

 contamination of food by tubercular material from the human 

 subject may occur. Alike when inhaled and when ingested, 

 tubercle bacilli may lodge about the pharynx and thus come to 

 infect the pharyngeal lymphoid tissue, tonsils, etc., tubercular 

 lesions of these parts being much more frequent than was 

 formerly supposed. Thence the cervical lymphatic glands may 

 become infected, and afterwards other groups of glands, bones, 

 or joints, and internal organs. 



The Specific Reactions of Tubercle Bacilli. — The tubercle 

 bacillus belongs to the group of organisms which do not to any 

 extent secrete soluble toxins, but which nevertheless produce 

 effects in the body at a distance from the site of actual prolifera- 

 tion. The origin of these effects is obscure, but there is abundant 

 evidence that, while the injection of dead bacilli tends to pro- 

 duce local lesions, the introduction of the disintegrated proto- 

 plasm of the bacillus can produce pathogenic effects of a toxic 

 character. Such disintegrated products (which may be looked 

 on as endotoxins), artificially prepared, were introduced by Koch 

 under the name of tuberculins, and the following are the chief 

 forms in use : — 



(1) Koch's Old Tuberculin.— -This consists of a six-weeks'-old culture of 

 tubercle bacilli in 5 per cent, glycerin bouillon, evaporated down to a 

 tenth of its original volume, killed by heat, and filtered. It thus contains 

 the products of macerated bacilli, substances (not destroyed by heat) 

 formed from the medium during the growth of the organism or extracted 

 from the bacilli by the glycerin,— and the remains of the medium. 



(2) Tuberculin-O.— Masses of living bacillary growth from surface 

 cultures on agar are dried m vacuo, ground in an agate mill, treated 

 with distilled water and centrifugalised ; the supernatant clear fluid is the 

 tuberculin. As it gave no cloudiness on the adddition of glycerin, Koch 

 concluded that it contained the glycerin soluble products present in the 

 "old tuberculin " and which were looked on as responsible for the necrotic 

 effects produced by the latter {vide mfra). 



(3) Tuberculin- R — The deposit in the preparation of tuberculin-0 is 



