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 win length, and are commonly slightly curved or 
bent at an angle; the diameter is 
about 0.2 4. 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 reds are usually soli- 
vary » but wud be united ee gina se Fie. 114. — Bacillus tuberculosis. 
in short chainscontaining 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 charaeters of this bacillus are extremely important 
for its differentiation and recognition in preparations of sputum, etc. 
Unlike most microérganisms 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 
