268 TUBEECULOSIS 



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 tubsrculosis and also a certain pro- 

 portion of cases of human tuberculosis. The description which 

 follows applies to the human type. 



Tubercle Bacillus (Human Type)— Microscopical Characters. 



— Tubercle bacilli are 



^^i"", ' -» minute rods which usually 



f S y*~ ' ) *"*■-, measure 2 - 5 to 3 - 5 fi in 



\ !'• '* w. length, and - 3 ft. in thick- 



i \ ' \^^ "^C ness, i.e., in proportion to 



I - - i O^j j4 their length they are com- 



.,-" \ '*> v ^v i V\\ paratively thin organisms 



\ \ \~\/. U~ I | ( Fi S s - 73 and 74 )- Some - 



^J . -r ^ jX ! times, however, longer 



/ A ^V. f \ ' f° rrns J up to 5 /^ or more 



. ^_ ' '^/""^v *?• / i Q length, are met with, 



\^ Vf i*/^ ^ >0 ' ;n * n cultures and in 



n \ %£*/ ^ e tissues. They are 



\ v straight or slightly curved, 



' and are of uniform thick- 



ness, or may -show slight 

 Fig. J3.-Tubercle bacilli of the human type, swe vj m „ at their ex . 

 from a pure culture on glycerin agar. . , & 



Stained with carbol-fuchsin. xlOOO. tremities. When stained 



they appear uniformly 

 coloured, or may present small uncoloured spots along their course, 

 with darkly stained parts between. There is no satisfactory 

 evidence that such appearances represent spore-formation, as some 

 have supposed ; and it has been shown that "beaded " bacilli have 

 no higher powers of resistance than those which stain uniformly. 

 The bacillus is seen in the " beaded " form when grown on 

 media containing sperm or olive oil (A. H. Miller). 



The bacilli in the tissues occur scattered irregularly or in 

 little masses. They are usually single, or two are attached end 

 to end and often form in such a case an obtuse angle. True 

 chains are not formed, but occasionally short filaments are met 

 with. In cultures the bacilli form masses in which the rods are 

 closely applied to one another and arranged in a more or less 

 parallel manner. Tubercle bacilli are quite devoid of motility. 



