THE TUBERCLE BACILLUS 273 



From these statements it will be seen that the disease in 

 animals presents great variations in character, and may differ in 

 many respects from that met with in the human subject. The 

 relations of the different forms of tuberculosis are discussed below, 

 but it may be stated here that two chief types of mammalian 

 tubercle bacilli are now recognised a human type which is the 

 common cause of tuberculosis in the human subject, and a bovine 

 type which produces bovine tuberculosis and also a certain pro- 

 portion of cases of human tuberculosis. The description which 

 follows applies to the human type. 



Tubercle Bacillus Microscopical Characters. Tubercle 

 bacilli are minute rods 



which usually measure -^/ **~ i '. 



2-5 to 3'5 /x in length, f f /T 'S* ) *^ 



and *3 ^ in thickness, '/ 



i.e., in proportion to their / -' 



length they are compara- / - 



tively thin organisms i "~ \ V " x NJ\\ \ \s \ 



' 



(Figs. 78 and 79). Some- , ^> U~ ' 



times, however, longer -<r ^ 



forms, up to 5 /x or more / A Vy t (' V\ 



in length, are met with, , i^, ' ^ '^X^-C *T' 



both in cultures and in l*)& 



the tissues. They are i 



straight or slightly curved, i * 

 and are of uniform thick- 

 ness, or may show slight 



^wpllhio- at thpir px FIG. 78. Tubercle bacilli of the human type, 

 swelling at from a pure culture on glycerin agar> 



tremities. When stained Stained with carbol-fuchsin. x 1000. 

 they appear uniformly 



coloured, or may present small uncoloured spots along their 

 course, with darkly stained parts between. In such a minute 

 organism it is extremely difficult to determine the exact nature 

 of the unstained points. Accordingly, we find that some ob- 

 servers consider these to be spores, while others find that it is 

 impossible to stain them by any means whatever, and consider 

 that they are really of the nature of vacuoles. Against their 

 being spores is also the fact that many occur in one bacillus. 

 Others again hold that some of the condensed and highly stained 

 particles are spores. It is impossible to speak definitely on the 

 question at present. We can only say that the younger bacilli 

 stain uniformly, and that in the older forms inequality in stain- 

 ing is met with ; this latter condition is, however, not found to 

 be associated with greater powers of resistance. 

 18 



