THE INFECTIOUS DISEASES. 249 



For purposes of simple recognition of the bacilli in sections it seems to the writer 

 usually better to have no color in the preparation other than that which the tubercle 

 bacilli possess. But it is often convenient to demonstrate the nuclei of the cells at the 

 same time, and this may be accomplished by staining lightly afterward with a dilute 

 solution of some color which will contrast with that of the bacilli, such as methylene 

 blue. 



In the examination of urine for the presence of the tubercle bacillus it is well to 

 collect the sediment by means of a centrifugal machine. In the examination of milk, 

 or other fat-containing fluids for tubercle bacilli, it is well, after the film has been 

 formed upon the cover glass and before staining, to rinse with chloroform followed by 

 alcohol, and this by water. 



Occasionally one finds in urine acicular crystalline bodies considerably resembling 

 the tubercle bacillus in size and shape, and retaining a red color after the decolorization 

 of the specimen. A careful study of the form, however, will suffice to prevent mistakes. 



The only other bacilli which are liable to be mistaken for the tubercle bacilli are 

 the bacillus of leprosy and the so-called smegma bacillus which sometimes occurs be- 

 neath the prepuce. The lepra bacillus may be distinguished from the tubercle bacillus 

 by the following differential staining process: If the lepra bacillus be stained for ten 

 minutes in a dilute alcoholic solution of fuchsin (five drops of saturated alcoholic solu- 

 tion of fuchsin to 3 c.c. of water), and then rinsed fora few seconds in a solution of 

 nitric acid (one part) in alcohol (ten parts), it will retain a red color, while under the 

 same treatment the tubercle bacillus remains uncolored. For differential stains for the 

 smegma bacillus see page 253. 



Varieties of Tubercle Bacilli. 



While many of the lower mammalia are susceptible to inoculations with tubercle 

 bacilli derived from man, the bacilli obtained from spontaneous tuberculous lesions in 

 the lower animals may present noteworthy variations in form and growth from the 

 human type. Thus Theobald Smith and others ' have pointed out marked peculiarities 

 of the bovine bacillus which would lead to the conjecture that though not a separate 

 species it is probably a distinct variety of the organism. 



Still more marked are the differences between mammalian and avian or bird tuber- 

 cle bacilli. Whether in this case also the differences are to be regarded as specific or 

 within the limits of variational sway is not yet clear. Although the subject is impor- 

 tant from the standpoint'of prophylaxis and interesting from the point of view of the 

 variations of bacteria under changes of environment, the attempts by experimental 

 means to convert one form into the other have thus far been too few to lead to definite 

 conclusions. 



Tuberculin. 



When the tubercle bacillus is grown on glycerinated nutrient broth certain meta- 

 bolic products are formed and pass into solution in the fluids. If after some weeks of 

 vigorous growth the germs are separated by filtration and the broth is concentrated by 

 evaporation, a dark-brown fluid results which is called tuberculin. This substance at 

 one time believed by many, and still by a few observers, to possess distinct curative 

 properties in certain forms of tuberculosis has assumed great economic importance on 

 account of its value as a diagnostic agent in bovine tuberculosis. For if administered 

 subcutaneously in small quantity to cattle , a marked temperature reaction follows in 

 tuberculous animals, while those which are sound are unaffected. The existence of 

 even very slight lesions may be detected in this way. In man also tuberculin has proved 

 of value in cases in which the efforts to establish a diagnosis by the usual method have 

 failed. 



ACTION OF DEAD TUBERCLE BACILLI. 



It has been found that tubercle bacilli which have been killed by 

 boiling or otherwise, when introduced into the body of the rabbit either 

 beneath the skin, or into the serous cavities, or into the blood-vessels and 



'See Walbach and Ernst, Jour, of Med. Research, vol. x., 1903, p. 313, bibl. ; also 

 Ravenel, Medicine, July and August, 1902; see also reference to Raveuel, p. 252. 



