EEPOET OF THE BUREAU OF ANIMAL INDUSTRY. 105 



description could be observed in these dead masses and the lung itself 

 usually emitted a putrefactive odor. 



In the interpretation of these sections under the microscope we 

 must be very careful in assigning any particular role to the bacteria 

 present. Most of them are there because of the previously existing 

 disease, which, so to speak, prepared the soil for them. In the second 

 place the bacteria, which are in reality the cause of the disease, may 

 be present only at a certain stage of the disease, being subsequently 

 destroyed as the lungs heal, or giving way to accidental forms as 

 the disease progresses. We must bear in mind with reference to 

 the second alternative that pathogenic bacteria must suffer by the 

 gradual changes which they themselves induce. Thus in the early 

 stages they undoubtedly live and multiply in the exudate which is 

 contained in the alveoli. When this becomes more and more con- 

 solidated, and as the ultimate bronchi are occluded by exudate, the 

 bacteria are being deprived of nutriment and oxygen. The tissue 

 dies, and with it the bacteria originally causing its death ; other bac- 

 teria more adapted to the conditions now prevailing get a foothold, 

 until the entire lung becomes a prey to many kinds of bacteria. 



A well-known illustration may be cited in support of these asser- 

 tions. The tubercle bacilli, which may be seen in sections of young 

 tubercles, cannot, as a rule, be found in the caseous mass which forms 

 later on in the center of the enlarging tubercle. Inoculations of 

 blood serum with such material are apt to prove failures, and if it 

 were not that inoculation into guinea pigs is almost invariably suc- 

 cessful we might presume that the bacilli had perished. The tact is, 

 the bacilli, finding no suitable conditions of growth in the caseous 

 mass, would perish if it were not that they have the capacity to form 

 spores under such circumstances. These spores, which may fail to 

 germinate on blood serum, find a more suitable medium in guinea 

 pigs, where they soon give rise to a generalized tuberculosis. 



The problem to be solved, therefore, was to isolate the specific bac- 

 teria which are the cause of the disease from the rest. The method 

 pursued was to introduce minute bits of diseased lung tissue beneath 

 the skin of rabbits and mice. If the specific bacteria are present 

 they will in all probability cause the death of the inoculated ani- 

 mals. They will then be found in one or more of the internal organs, 

 from which they can be obtained free from the other bacteria. These 

 will remain restricted to the place where they were deposited. This 

 method of obtaining disease germs has been used by other investi- 

 gators, more particularly by Schiitz, in the study of swine plague in 

 Germany, and more recently in investigations or infectious pneumo- 

 nia in horses. A re'sume' of this work on the bacterium causing 

 swine plague has been given in the preceding report of the Bureau in 

 connection with the preliminary investigations made last year of 

 American swine plague. The facts in the case are briefly as follows: 

 It was found that in the majority of cases, when bits of diseased lung 

 tissue were placed beneath the skin of rabbits and mice (or simply 

 rubbed into any slight prick made on the ear with a lancet), a septi- 

 caemia appeared with which the bacteria described in the preceding 

 report were always and exclusively associated. Rabbits are more 

 susceptible than mice, and die in from one to four days after inocula- 

 tion. By this means pure cultures of the same bacteria were obtained 

 from most of the cases reported in the preceding pages. This was 

 therefore the only microbe present which was capable of destroying 

 the smaller experimental animals. 



