120 OUTLINES OF BACTERIOLOGY 



commonest among cattle, but is also quite common among pigs, horses, 

 dogs, cats, and fowls. Sheep and goats appear to be immune to this 

 disease. Whether the species causing tuberculosis in the human 

 subject is absolutely identical with that causing the same disease in 

 cattle is still undecided, for though Koch maintained at the Tubercular 

 Congress of 1901 that they were quite distinct species, the general 

 opinion of the medical world is against him, maintaining that the same 

 species causes the disease both in man and in cattle. The subject is 

 still under investigation. Its importance is obvious when we reflect 

 on the quantity of food in the shape of milk that we obtain from cows. 

 With regard to avian tuberculosis, there is reason to believe that 

 the human subject is not susceptible to the bacillus obtained from a 

 tubercular fowl, as the following results show : 



1. Human tubercle bacilli produce no effect when a solution con- 



taining them is injected into birds. 



2. Human tubercle bacilli produce acute tuberculosis when intro- 



duced in the same way into dogs. 



3. Avian tubercle bacilli produce acute effects on birds but not 



on dogs. 

 It seems, therefore, extremely probable that the avian Bac. tuberculosis 

 is a different variety to the kind found on man and dogs, though of 

 course belonging to the same species. Unfortunately, however, there 

 is every reason to believe that the avian is only a modification of the 

 human variety, for by gradually accustoming the latter to the con- 

 ditions under which the avian variety usually grows, it can be induced 

 _ ^ to assume all the characteristics of the 



\^ avian variety. The rods of Bacillus 



* ^P^0 tuberculosis are very small, being only 



-^ 25-3-5/i long and -3 /x. broad (Fig. 74), 

 though sometimes they are longer — up 

 to 5 /x or more. They are usually 

 Fig, 74.— Bac. tuberculosis. Straight, but also Sometimes slightly 



curved. These bacilli take up stains 

 very slowly, so special methods must be employed for colouring 

 them. When they have taken up a stain they retain it tenaciously ; 

 thus, whilst a 20 per cent, solution of sulphuric acid immediately 

 takes away the colour from most stained bacteria, in this case the 

 decolorising agent must be allowed to act a long time before the 

 colour is removed. Hence, in a mixture of tubercle bacilli and 

 other bacteria it is possible to make a preparation in which the 

 former only are stained. The tubercle bacillus, however, is not the 



