BACILLUS TUBERCULOSIS. 265. 



tuberculous affections were examined ; here, however, 

 the characteristic bacilli were invariably absent. 



From all these microscopical investigations it follows 

 with certainty that the tubercle bacilli occur constantly 

 and exclusively in tuberculosis, that they precede the 

 pathological alterations which are peculiar to tubercu- 

 losis both in place and time, and that their number, their 

 appearance, and their disappearance, stand in direct 

 relation to the course of the disease. These results 

 evidently imply an etiological relation between the 

 tubercle bacilli and the tubercular process. 



The cultivation of the tubercle bacilli was also ac- Cultivation of 

 companied, like their microscopical demonstration, Bacilli. 

 by special difficulties, and required new and special 

 methods. No growth occurred on the ordinary nutrient 

 media. The tubercle bacilli evidently lead a parasitic 

 existence, and saprophytic growth can only occur when 

 the conditions under which they flourish in the animal 

 body are copied as accurately as possible ; as also the 

 whole development of the tubercular process only goes 

 on very slowly we can only expect a difficult and slow 

 growth of the bacilli, even under conditions which 

 to some extent correspond with those which are present 

 i;i the animal body. As a matter of fact Koch only suc- 

 ceeded in obtaining cultivations when he employed 

 blood serum, gelatine, or solidified blood serum kept at 

 the body temperature, and when also these nutritive 

 media were so prepared that they could be kept for 14 

 days or longer at the body temperature without drying 

 up, or becoming contaminated with other organisms. 

 It was found that blood serum kept in test tubes, and 

 sterilised, was the most suitable material, especially 

 when the tubes were kept in an oblique position while 

 the blood serum was solidifying, and thus a large 

 surface was presented for the growth of the cultivation. 

 (More accurate details as to the preparation of this 

 nutrient medium will be given in the chapter on 

 " Methods.") During solidification of the blood serum 

 the condensed water collects in large drops at the lowest 

 part of the oblique surface of the serum, and this reser- 



