KOCH DISCOVERS THE CAUSE OF TUBERCULOSIS 439 



tuberculosis. As each animal died, Koch examined its tissues and 

 always found tubercles and the slender rods. Then he examined 

 tissue after tissue of healthy animals and found no tubercles. .. 



He was not yet positive that he had discovered the tubercle 

 bacilli. He decided that he must first cultivate them outside the 

 body of animals and then introduce the cultivated organisms into 

 healthy animals. If the inoculated animals became tubercular, it 

 would indicate the presence of the organisms. He made every 

 kind of broth then known for the cultivation of germs. He kept 

 some of his tubes and bottles' at the temperature of the room, 

 some at the temperature of a man's body, and others at fever 

 temperature. He infected his broths with portions of the in- 

 fected lungs of guinea pigs. No tubercular organism grew in 

 any of the broths, regardless of surrounding temperatures. 



Undaunted, he thought he would try tissue extracts for his 

 media. Possibly tissues had peculiar materials that were essential 

 for the growth of the tubercle bacillus. He obtained serum from 

 blood, mixed it with agar, heated it to make it set, and placed it 

 on slants in test tubes in order to get long, flat surfaces on 

 which to grow the bacilli. He streaked the serum-agar with a bit 

 of the infected lung of a guinea pig that 

 had just died of tuberculosis. Then he 

 placed his tubes in an incubator at the 

 same temperature as that of the guinea 

 pig's body. On the fifteenth day the 

 serum jelly was covered with tiny glisten- 

 ing specks. When magnified with a hand 



lens, they appeared as dry, tiny scales. ' BacUrium 



He stained and mounted one of the scales T he rod-shaped structures, 



under the microscope and found the same L^tcmt fiTca^S 



bacillus which he had discovered in the lung berculosis - 



of tubercular victims. He had grown tubercle bacilli outside the 



body of an organism. He then inoculated animals with the bacilli 



