BACILLI IN CHRONIC INFECTIOUS DISEASES. 469 



ous in tuberculous growths of recent origin, and often cannot be 

 demonstrated, by microscopical examination, in caseous material 

 from the centre of older nodules. But such material, when inocu- 

 lated into susceptible animals, gives rise to tuberculosis, and the 

 usual inference is that it contains spores of the tubercle bacillus. 



Morphology. The tubercle bacilli are rods with rounded ends, 

 of from 1.5 to 3.5 ^ in length, and are commonly slightly curved or 

 bent at an angle ; the diameter is 

 about 0.2 //. In stained preparations 

 unstained portions are frequently 

 seen, which are generally believed to 

 be spores, but this is by no means 

 certain. From two to six of these 

 unstained spaces may often be seen 

 in a single rod, and owing to this al- 

 ternation of stained and unstained 

 portions the bacilli may, under a low 

 power, be mistaken for chains of mi- 

 crococci The rods are usually soli- 

 tary, but may be united in pairs, or 



. V i * Fj - m - Bacillus tuberculosis. 



m short chains containing three or four x 1,000. From a photomicrograph, 

 elements. In old cultures irregular 



forms may be observed, the rods being sometimes swollen at one 

 extremity, or presenting the appearance of having a lateral bud-like 

 projection involution forms. 



The staining characters of this bacillus are extremely important 

 for its differentiation and recognition in preparations of sputum, etc. 

 Unlike most microorganisms of the same class, it does not readily 

 take up the aniline colors, and when stained it is not easily decolorized, 

 even by the use of strong acids. The failure to observe it in tuber- 

 culous material, prior to Koch's discovery, was no doubt due to the 

 fact that it does not stain in the usual aqueous solutions of the aniline 

 dyes. Koch first recognized it in preparations placed in a staining 

 fluid to which an alkali had been added solution of methylene blue 

 with caustic potash ; but this method was not very satisfactory, and 

 he promptly adopted the method devised by Ehrlich, which consists 

 essentially in the use of a solution of an aniline color fuchsin or 

 methyl violet in a saturated aqueous solution of aniline oil, and de- 

 colorization with a solution of a mineral acid nitric acid one part to 

 three parts of water. 



The original method of Ehrlich gives very satisfactory results, 

 but various modifications have since been proposed, some of which 

 are advantageous. The carbol-fuchsin solution of Ziehl is now 

 largely employed ; it has the advantage of prompt action and of 



