KOCH. 63 



tissues were rendered transparent. So completely was this 

 accomplished, that the animal tissues were practically gotten 

 out of the way of the observation of the organisms. His 

 plans have now been tried by a large number of investigators, 

 and have stood the test of adverse criticism. By this new 

 plan of work it was soon demonstrated that the normal tissues 

 of men and animals are entirely free from these organisms ; 

 and that the bodies before thought to be such by Billroth, 

 Thiersch, and many others, were, in fact, the granular con- 

 stituents of the blood. Billroth and others, after experiment- 

 ing with the new process, have fully admitted the demonstra- 

 tion. And the point may be considered as fully settled. 



Dr. Koch now began his work by the then common plan 

 of injecting septic blood into the tissues of animals. At first, 

 his results were similar to those common with this mode of 

 experimentation. Too much of the material killed, by direct 

 poisoning, while a small amount induced disease. Even the 

 slightest scratch of a blade that had been dipped into the 

 blood of the dead house mouse on which he was experimenting 

 produced the disease and death, in the usual time, upon a 

 second mouse, i. e., from forty to sixty hours. The organism 

 found in the blood of the dead animals was a very minute 

 bacillus, which remained constant and pure. An effort, during 

 these experiments, to inoculate field mice with the bacillus 

 cultivated in this way in the house mice, failed entirely. 

 They proved to be in no wise susceptible to the disease. 



Now, Dr. Koch, in his examinations of the tissues of the 

 dead house mouse of the original experiment, had found at 

 the point of injection an organism entirely different from 

 that found in the blood, and which he got rid of in the injec- 

 tions made afterward ; having obtained these in as perfect 

 isolation as he could, by the means at that time devised, and 

 tried them on the field mice, he was surprised to find that 



