STAINING PECULIARITIES. 215 



ing of rods a little clubbed at one extremity or slightly 

 bulging at different points, may be detected. It varies 

 in length — sometimes being seeu in very short seg- 

 ments, again much longer. On an average its length is 

 seen to vary from 2 to 5 ^. 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. 



These rods usually present, as has been said, an 

 appearance of alternate stained and colorless portions. 

 It is the latter portions which are believed to be the 

 spores of the organism, though as yet no absolute proof 

 of this opinion has been established. 



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. 



Staining Peculiarities. — A peculiarity of this 

 organism is its behavior toward staining reagents, and 

 by this means alone it may be easily recognized. The 

 tubercle bacilli do not stain by the ordinary methods. 

 They possess some peculiarity in their composition 

 which renders them more or less proof against the 

 simpler dyes. It is therefore necessary that more ener- 

 getic and penetrating reagents than the ordinary watery 

 solutions should be employed Experience has taught 

 us that certain substances not only increase the solu- 

 bility of the aniline coloring substances, but by their 

 presence the penetration of the coloring agents is very 

 much increased. These substances are aniline oil and 

 carbolic acid. They are both present in the point of 

 the solutions to about saturation. (For the exact 

 proportions see chapter on Staining Reagents.) 



Under the influence of heat, these solutions are seen 



