§ XIII.] Bacillus of tubercle. 155 



have not been successful ; the cardinal points for the tempera- 

 tures of vegetation are those given on page 51. 



The Bacillus of tubercle offers a somewhat high degree of 

 resistance to injurious influences from without, and is thus able 

 to preserve its powers of infection. It can bear temperatures 

 approaching the boiling point, though it is soon killed if it is 

 heated in a thoroughly moist condition. It was not affected by 

 desiccation during a period of 186 days, or by being kept in 

 putrefying sputum for 43 days. The experiments on its powers 

 of resistance have chiefly been made with sputum containing the 

 Bacilli. No attempt has been made to determine precisely how 

 far these powers of endurance arc confined to the spores or 

 belong also to the vegetative rods, but our experience in other 

 cases would lead us to suppose that they belong chiefly to the 

 spores. 



These facts taken together give a satisfactory explanation of 

 the appearance of tuberculosis as the result of infection with the 

 Bacillus. Every one knows how widely spread the disease is, 

 even if we think only of pulmonary tuberculosis ; a seventh part 

 on an average of the deaths of human beings are caused by this 

 form of the disease. The Bacillus is generally present, capable 

 of development and in a virulent condition, in the excretions of 

 those suffering from tuberculosis. The expectorations of con- 

 sumptive persons often during months and years must be espe- 

 cially but by no means exclusively taken into account in this 

 connection. The Bacillus was absent from 44 only of the 982 

 specimens of sputa examined by Gaffky. It is clear that the 

 Bacillus is communicated in large numbers to these excretions, 

 and that when they dry up, it must be disseminated with the 

 dust and in other ways. There is therefore abundant oppor- 

 tunity for infection in the ordinary intercourse of human beings. 

 We need not enter more minutely into this point, and to discuss 

 the mode in which the Bacillus spreads in the body which has 

 once become infected would also lead us too far into medical 

 details. That a good deal depends on the susceptibility of the 



