ETIOLOGY 131 



cally unknown. The Western steers that are killed in the 

 slaughter houses of Kansas City, Omaha, Sioux City and 

 Chicago are practically free from this disease. In many locali- 

 ties, especially where there is an extensive interchange of 

 animals, a large percentage of the herds are more or less 

 affected, 



§109. Etiology. Tuberculosis is caused 

 by a rodshaped organism known as Bacter- 

 ium tubcniilosis. It was discovered by Rob- 

 ert Koch in 1882. Schiiller and Toussaint 

 had previously studied growths which .seem, 

 from the results of their inoculation experi- 

 ments, to have been this organism. The bac- 

 terium of tuberculosis is a slender rod-shaped Fig 23. 

 organism with rounded ends, from 2 to 5/< in terinvi of iubcr- 

 length and from o 3 to o.5/( broad. The rods '^"''«"- 

 are straight or slightly curved, and occ\ir singly, in pairs or 

 in small bundles. Frequently they cross one another. They 

 do not produce spores, but vacuoles are often observed and 

 branching forms have been described. 



The bacterium of tuberculosis is readily- cultivated on arti- 

 ficial media such as blood serum, glycerinated agar and bouil- 

 lon, and potato after it has been adapted to such artificial con- 

 ditions,* It is, however, not easy to cultivate it directly from 



*To accomplish this necessitates a very special and careful proce- 

 dure. Dr. Theobald .Smith, of Harvard University (Jour, of Exp. Med., 

 Vol. III., 1S98, p. 45r), has the credit of formulating a method by com- 

 bining details in such a manner that the procuring of cultures is, in 

 most cases, possible. Dog serum is used. The method, as he gives it, is 

 as follows, viz. : — 



"The dog was bled under chloroform and the blood drawn from a 

 femoral artery, under aseptic conditions, through sterile tubes direct!}' 

 into sterile fiasks. The serum was drawn from the clots with sterile 

 pipettes and either distributed at once into tubes or else stored with 0.25 

 to o 3 per cent chloroform added. Discontinued sterilization was ren- 

 dered unnecessary. The temperature required to produce a sufficiently 

 iirm and yet not too hard and dry serum is for the dog 75° to 76° C. For 

 horse serum it is from 4° to 5° lower. The serum was set in a thermo- 

 stat into which a large dish of water was always placed to forestall any 

 abstraction of moisture from the serum. About 3 hours suffice for the 



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