TEXT-BOOK OF BACTERIOLOGY. 225 



body, and by transmissions from artificial cultures, we know that 

 the black-leg bacillus is the specific and sole cause of the black-leg 

 malacty. We must now ask how the symptoms of the disease and 

 the circumstances of its appearance are to be explained in connec- 

 tion with the qualities of the micro-organism which excites it. 



Although we are still unable to give an answer that shall be 

 satisfactory in all respects, yet we may say, with a high degree of 

 probability, that under natural conditions the infection of animals, 

 the spread of the disease, is owing to the penetration of the bac- 

 teria through small wounds, particularly in the extremities, the 

 thighs, etc., and their further development in the subcutaneous 

 tissue, in which they cause the peculiar changes of which we have 

 spoken. The two other ways by which micro-organisms can ob- 

 tain entrance to the body, the intestinal canal and the lungs, may 

 also occasionally be taken by these bacilli, 3 r et neither experiment 

 nor observation has yet yielded satisfactory proofs of such being 

 the case. 



The black-leg bacillus, in contrast to the anthrax bacillus, is 

 scarcely able to exist outside the bodies of animals, for on account 

 of its anaerobic character it speedily dies in contact with the air. 

 The task of preserving, spreading, and propagating the virus is 

 probably performed by the spores exclusively. 



They begin to form in the body soon after death has taken 

 place, and the flesh of the dead victims contain large supplies of 

 them. If the animals are skinned, cut up, etc., the ground is in- 

 fected at the place in question by means of escaping tissue-juice, 

 and so gives rise to a black-leg locality, and even if the bodies are 

 not opened, the bloody serous fluid peculiar to the disease may 

 nevertheless infect the place where the animals have stood, making- 

 it dangerous for healthy animals that may afterward come there. 

 It is, therefore, easy to understand that the malady adheres to 

 certain districts, that it often appears suddenly epidemically, that 

 it sometimes disappears as suddenly (when the cattle no longer 

 visit the place of infection), that it is generally confined to the 

 summer months, etc. 



It is to be noted that young and also old animals are but little 

 susceptible; as a rule, those from one to three years old are at- 

 tacked. Further, the young of animals which have been protec- 

 tively inoculated usually inherit the immunity of their parents. 



IV. TUBERCLE BACILLUS. 



When we consider that almost one-seventh of all deaths are due 

 to tuberculosis, and that this disease occurs very frequently among 



