METHODS OF INFECTION. 271 



tuberculosis. An atomizer is employed to bring the organ- 

 isms into a finely divided condition. It need hardly be 

 stated that this method is extremely dangerous to the_ 

 operator. In fact, it is the most dangerous procedure in the 

 entire bacteriological technique, and, for that reason, it is 

 but rarely resorted to. 



The intra-tracheal injections have been extremely valu- 

 able in the study of diphtheria. The method is simple, and 

 is carried out in much the same way as when making injec- 

 tions into the jugular vein. The animal is fixed on its back 

 and anesthetized. After removal of the hair, and after 

 local disinfection, a median incision is made. The trachea 

 is then gradually exposed, making use of a grooved sound. 

 When fully exposed, it is well to pass the sound under the 

 trachea, thus placing this on the stretch. The needle is 

 then easily inserted between the cartilaginous rings. A 

 few scratches should be made with the needle on the inside 

 of the trachea, in order to give the organism a foot-hold. 

 The amount of fluid injected should be relatively small. 

 Frequently, the material is introduced into the trachea by 

 means of the platinum wire. In that case, one or two 

 tracheal rings are cut and the mucous membrane is slightly 

 injured by scratching, or by means of a hot wire. The 

 wound is then closed, employing the usual precautions 

 . against accidental infection. 



10. Infection of the alimentary canal. A large number 

 of diseases of man and animals are due to infection along 

 this tract. Hence, in establishing the relation of suspected 

 organisms to such diseases, it is desirable to do so by fol- 

 lowing, as nearly as possible, the natural mode of infection. 



The usual procedure is to administer the bacterial fluid 

 with the food or drink. When the animal refuses to par- 

 take it is necessary to employ a stomach-tube. A rubber 

 tube, about five mm. in diameter, will answer this purpose. 

 It should be connected with a small funnel. In order to 



