BACILLI IN CHRONIC INFECTIOUS DISEASES. 



397 



about half a minute in dilute nitric acid (ten per cent) ; then wash 

 out color in sixty-per-cent alcohol ; counter-stain for two or three 

 minutes in a saturated aqueous solution of methylene blue ; dehydrate 

 with absolute alcohol or with aniline oil ; clear up in oil of cedar, 

 and mount in xylol balsam. If the aniline-water-methyl-violet solu- 

 tion has been used for staining the bacilli a saturated solution of 

 vesuvin may be used as a contrast stain. 



Biological Characters. A parasitic, aerobic, non-motile ba- 

 cillus, which grows only at a temperature of about 37 C. Is also a 

 facultative anaerobic (Frankel). 



The question as to spore formation has not been definitely deter- 

 mined. It has been generally assumed that the unstained spaces 

 which are frequently seen in the bacilli are spores ; and the fact that 



FIG. 118. Section through a tuberculous nodule in the lung of a cow, showing two giant cells 

 containing tubercle bacilli, "x 950. (Baftungarten.) 



caseous material in which a microscopical examination has failed to 

 demonstrate the presence of bacilli may produce tuberculosis, with 

 bacilli, when inoculated into guinea-pigs, has been explained upon the 

 supposition that this material contained spores. But a few bacilli 

 present in such caseous material might easily escape detection. As 

 pointed out by Frankel, the oval spaces in stained specimens have 

 not the sharply denned outlines of spores. Moreover, the bacilli, when 

 examined in unstained preparations, do not contain corresponding re- 

 fractive bodies, recognizable as spores. And when the bacilli are 

 stained by Gram's method the protoplasm is often contracted in the 

 form of little, spherical stained masses, while the unstained spaces 

 are larger and no longer have the oval form presented in rods stained 

 by Ehrlich's method. The great resisting power of the bacillus to 

 heat and to desiccation has been supposed to be due to the presence 



