TUBERCULOSIS. 215 



cover glass on to the surface of a colony as it grows on 

 the serum, and then removing it without sliding it in any 

 way and staining by any of the usually recognized methods. 



As regards the conditions under which this organism 

 exists, we have already seen that a certain amount of 

 moisture is absolutely necessary for its growth, and Paw- 

 lowsky was able to cultivate it, even on potato, when he 

 took the precaution to keep a considerable quantity of 

 moisture in contact with the growing organism, and watched 

 the potato for a considerable length of time, the growth not 

 being visible to the naked eye for three weeks or a month. 

 Koch's great difficulty in his earlier experiments was, as we 

 have already seen, to obtain a substance which, in addition to 

 containing all the other elements necessary for the nutrition 

 of the bacillus, would remain sufficiently moist for the 

 requirements of the bacillus when exposed to a pretty high 

 temperature for a considerable length of time. As regards 

 temperature, it was found that, although there was an actual 

 cessation of growth and development below 28°.or 29° C, 

 on blood serum the organism might be exposed to very low 

 temperatures for a considerable length of time without 

 losing its power of again becoming active when returned to 

 favourable environments. It grows best at about 37° C, 

 but as soon as the temperature rises beyond 38° C. the 

 development of the bacillus in the cultivation tubes begins 

 to diminish in activity, and at 40° C. (Koch originally gave 

 this limit as 42° C.) it ceases entirely to grow and multiply. 

 It is a remarkable fact that the temperature at which the 

 bacillus develops best is exactly that of the human body 

 (37.8° C). In the cow and the horse, where the other con- 

 ditions must also be favourable, the temperature is about 

 38.3° C, in the calf a little over 39° C, whilst in the hen we 

 have the maximum temperature of 40° C. 



Klein, in a series of experiments reported in 1886, finds 

 that it is possible to inoculate successfully with tubercle 

 taken from the human subject, also that it is possible to 

 inoculate from a guinea-pig to a cow, but when inoculation 

 is made from a cow to a fowl the experiment breaks down, 

 and there is no tubercle produced, so that, not only can one 

 modify the activity, and the power of growth of these 

 bacilli outside the body by altering the temperature at which 

 they grow, but it is also within the range of possibility 



