BIOLOGY AND MORPHOLOGY. 189 



of its slow growth, and, further, because the associated organ- 

 isms outgrow the tubercle germ and thus suppress its growth. 

 Except for experimental purposes and for the manufacture 

 of the various tuberculin products, it is not necessary to make 

 cultures of the bacillus ; but when a pure culture is desired, 

 a special line of procedure must be followed : 



A number of animals which are very susceptible to tubercular 

 infection, such as guinea-pigs, are inoculated with the tuber- 

 cular material, at intervals of one day. Within about four or 

 five weeks the animal inoculated first will die from tubercu- 

 losis, which is confirmed at the autopsy. One of the other 

 animals is then killed, and under the strictest antiseptic pre- 

 cautions its abdomen and peritoneal cavity are opened. With 

 sterile instruments the spleen is brought into view, as this is 

 the organ which usually is affected most by the tubercular 

 process. Examine the surface of the spleen for a tubercular 

 nodule and excise it with sterile scissors. This nodule is then 

 compressed between sterile glass slides and transferred to 

 tubes containing blood-serum. Seal these tubes with rubber 

 caps and place them in the incubator. It is perhaps needless 

 to caution the operator never to allow his hands to come in 

 contact with any tubercular material, as infection is extremely 

 liable to occur. Everything should be handled with sterile 

 instruments, so as to avoid contamination of the pure culture 

 of the bacillus. 



Pure cultures may also be obtained directly from tuber- 

 cular sputum. The patient's mouth is first thoroughly disin- 

 fected. He is then instructed to expectorate into a sterile 

 Petri dish. This sputum is subjected to repeated washings 

 with sterile water until all the bacteria which may have been 

 lodged on the surface of the sputum are removed. The mass 

 is next carefully separated with a sterile needle, its centre 

 removed and placed on glycerin-agar or blood-serum. After 

 two or three weeks small grayish-white scaly colonies appear 

 on the medium. These gradually coalesce to form an irregu- 

 lar scaly membranous growth. 



After the bacillus has become habituated to being grown 

 outside the body, it is preferable to perpetuate the cultures 

 on glycerin-agar. The growth on glycerm-agar resembles 



