ORGANISMS WHICH PRODUCE DISEASE 79 



of destruction and proliferation are produced the nodules or 

 tubercles from which the disease has received its name. These 

 tubercles vary from minute microscopic nodules to large 

 potato-like masses, the latter variety tending to be especially 

 found in cattle. Extensive softening of tuberculous areas 

 may occur, giving rise to tuberculous or " cold " abscesses ; 

 or, as in the case of the lungs, where the contents can be 

 evacuated through the bronchial tubes, producing phthisical 

 cavities. Within the body, unless the disease is arrested, it 

 tends to spread progressively, both locally and to distant 

 parts along the lymphatics and blood-vessels. Lymphatic 

 glands, serous cavities, as well as the lungs and other tissues, 

 are specially liable to invasion, and in the young subject the 

 bones and joints, as well as the glands, etc., are very commonly 

 attacked. 



In any given case, the two antagonistic processes of toxic 

 destruction and tissue proliferation may occur each in varying 

 degree, according as the bacilli or the tissues gain the upper 

 hand. General toxic effects such as emaciation and sweating, 

 etc., and symptoms depending on the site of the lesion, such 

 as cough and spit, haemorrhage, and the like, are in some 

 cases familiar evidences of the disease. 



Different Strains or Types of the Tubercle Bacillus. 

 Almost all animals are, or may be rendered, susceptible to 

 tuberculosis in varying degree. Man and cattle are extremely 

 liable to the disease, as are also various domesticated and tame 

 animals. From 15 to 20 per cent, of cattle killed in slaughter- 

 houses in this country are reported to be tuberculous, and of 

 pigs from 2 to 3 per cent. With more thorough inspection it 

 is probable that these figures would be found to be considerably 

 under the mark. Many animals which do not in natural sur- 

 roundings take the disease often die of it under artificial 

 conditions, e.g. in menageries and zoological gardens, or when 

 kept as domestic pets. The disease is less common among 

 horses and sheep. Laboratory animals such as guinea-pigs and 

 rabbits, though not naturally subject to the disease, are ex- 

 tremely susceptible to artificial inoculation. The disease is also 

 found in birds, especially in poultry fowls, turkeys, pigeons, 

 and hand-reared game-birds such as pheasants and is then 

 known as Avian tuberculosis. It or at all events a closely- 

 allied disease may even be found in reptiles, amphibians, and 

 fishes, and a similar disease has been found in certain inverte- 

 brates. It is therefore a question of vital importance to deter- 

 mine whether the bacilli producing tuberculous diseases in 

 animals are or are not identical with one another, and are 

 capable of infecting man, An enormous amount of work has 



