THE TUBERCLE BACILLUS. 227 



is met with as a chronic disease of the lymphatic glands, 

 the so-called " scrofula " of pigs. Tubercular lesions in the 

 muscles are less rare in pigs than in most other animals. 

 In the horse the abdominal organs are usually the primary 

 seat of the disease, the spleen being often enormously 

 enlarged and crowded with nodules of various shapes and 

 sizes ; sometimes, however, the primary lesions are pulmonary. 

 In sheep and goats tuberculosis is a rare occurrence, 

 especially in the former animals. It also occurs spon- 

 taneously in dogs, cats, and in the large carnivora. It 

 is also sometimes met with in monkeys in confinement, and 

 leads to a very rapid and widespread affection in these 

 animals, the nodules having a special tendency to soften 

 and break down into a pus-like fluid. 



Tuberculosis in fowls (avian tuberculosis) is a common 

 and very infectious disease, nearly all the birds in the 

 poultry-yard being sometimes affected. The relation of 

 avian to mammalian tuberculosis is discussed below. 



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 tubercle nodules may be of so large 

 a size, e.g., in the horse and ox, as to be described as 

 sarcoma-like ; they may be tough and firm, with little or 

 no caseation, or they may be softened in the centre, more 

 resembling abscesses, or again there may be an eruption of 

 very minute granulations. However different their naked- 

 eye appearances may be, they are built up histologically on 

 the same plan, and of greater importance still is the fact 

 that they are all produced by the tubercle bacillus. An 

 account of the lesions experimentally produced will be given 

 later. 



Tubercle Bacillus Microscopical Characters. Tu- 

 bercle bacilli are minute rods which usually measure 2.5 to 

 3.5 /A in length, and .3 //, in thickness, i.e. in proportion to 

 their length they are comparatively thin organisms (Figs. 

 62 and 63). Sometimes, however, longer forms, up to 5 /^ 

 or more in length, are met with, both in cultures and in 



