BACTERIUM TUBERCULOSIS 445 



described. It varies in length sometimes being seen in 

 very short segments, again much longer, though never as 

 long threads. Usually its length varies from 2 to 5ju. It 

 is commonly described as being in length about one-fourth 

 to one-half the diameter of a red blood corpuscle. It is 

 very slender. (See Fig. 74.) 



These rods usually present, as has been said, an appear- 

 ance of alternate stained and colorless portions. At times 

 these colorless portions are seen to bulge slightly beyond 

 the contour of the rod, and in this way give to the rods the 

 beaded appearance so commonly ascribed to them. These 

 oval colorless areas were at one time thought to be spores. 

 A number of competent observers have expressed the opinion 

 that the rods which we see in tubercular lesions and which 

 we call bacillus tuberculosis are not, strictly speaking, 

 bacilli, but are fragments or developmental phases of a more 

 highly organized fungus possibly related to the strepto- 

 thrices or actinomyces. The point cannot now be decided. 



Staining Peculiarities. A peculiarity of this organism is 

 its behavior toward staining reagents, and by this means 

 alone it may easily be recognized. The tubercle bacillus 

 does not stain by the ordinary methods. It possesses a 

 peculiarity in its composition that renders it proof against 

 the simpler staining processes. It is therefore necessary 

 that more energetic and penetrating reagents than the 

 ordinary watery solutions be employed. Experience has 

 taught us that certain substances not only increase the 

 solubility of the aniline dyes, but their penetration as well. 

 Two of these are aniline oil and carbolic acid. They are 

 employed in the solutions to about the point of saturation. 

 (For the methods of staining B. tuberculosis see Chapter 

 on Staining.) 



