TEXT-BOOK OF BACTERIOLOGY. 155 



Third, the respiratory organs can afford entrance to the bac- 

 teria. Although the body has arrangements for excluding and 

 repelling foreign matters, especially in the upper part of the respi- 

 ratory tract, yet they are only able to perform this office within cer- 

 tain limits. 



Numerous experiments, especially those of H. Buchner, leave no 

 room for doubt that bacterial germs may be received by the organs 

 of respiration, may be inhaled, and settling on the uninjured sur- 

 face of the mucous membrane of the lung, may there produce a 

 general infection of the organism. 



Our artificial so-called methods of infection have been formed 

 after these natural models. 



The first of them is simple inoculation. By this we understand 

 a slight lesion of the cutis, to which the virus is applied, and whence 

 it is distributed over the body, chiefly by the circulating fluids. It 

 is rather difficult to perform this in the case of mice; in the ear, 

 however, it is sometimes possible to effect so slight a cut into the 

 outside skin that this alone is affected without injuring the tissue 

 beneath it. 



More frequent is the use of subcutaneous application.' Here the 

 subcutaneous cellular tissue is the place of deposit for the micro- 

 organisms, and the currents of the blood aid in their further dis- 

 tribution. With mice an incision is made above the root of the 

 tail, the skin being carefully separated from the subcutaneous tissue 

 by means of a scalpel or lancet-shaped needle, and thus a small 

 cavity formed, into which the inoculating material may be intro- 

 duced with a platinum wire. Or a portion of the skin on the back 

 is undermined with the scissors or forceps, and in this cavity the 

 silk thread charged with bacteria, the portion of tissue from an- 

 other animal, etc., is placed. 



With Guinea-pigs the abdominal region is more commonly 

 selected. The hair is removed at the spot chosen, a portion of skin 

 is nipped and raised with the forceps, a transverse cut is made with 

 the scissors, and a pouch is formed with the blunt-pointed scissor- 

 blade. Care must be taken to cut through the muscular layer, 

 which lies immediately beneath the skin, since otherwise the mat- 

 ter may fail to reach the inner tissues. With rabbits one proceeds 

 similarly, but any method is applicable and suitable by which an 

 entry is effected to the subcutaneous tissues without serious injury 

 to the animal. The operation must always be performed so care- 

 fully that but little blood flows from the wound, otherwise there is 

 danger of the inoculating material being washed away and res- 

 dered inefficacious. 



