METHODS OF INFECTION. 261 



or less difficult surgical operation. They include, moreover, 

 all the known avenues of infection whereby man and 

 animals contract disease. The several methods employed 

 will now be described as briefly as possible. The guinea- 

 pig, rabbit and white mouse are the animals employed most 

 often. The white rat, house mouse, pigeon, chicken, dog 

 and others are used less frequently. 



1. Cutaneous application. Bacteria cannot penetrate 

 into the interior of the body through an unbroken skin or 

 mucous membrane. Nevertheless, a person may become 

 infected, without any visible injury, as sometimes occurs 

 when making operations on the living, or when making 

 post-mortem examinations of the dead. In such cases, the 

 existence of microscopic abrasions or fissures, or an uncon- 

 scious puncture is looked upon as an explanation of the 

 infection. The organism to be tested in this manner is 

 rubbed into the skin as much as possible with the aid of 

 some fat or vaselin. In the study of pus-producing organ- 

 isms results have been obtained by this method which are 

 strictly analogous to conditions observed in man. The 

 production of boils or felons has thus been brought about 

 experimentally. 



2. Subcutaneous application or injection. By application 

 is meant, in this case, the introduction of the organism 

 through an opening in the skin: This is made by means of 

 a lance, or pair of scissors. Through the small nick, thus 

 made, the platinum wire or other instrument, laden with 

 the organism, is inserted into the subcutis and the material 

 is spread about as much as possible. This method may be 

 considered as analogous to the ordinary wound infection in 

 man and animals. A puncture with a sharp piece of wood, 

 glass or metal enables the introduction of the organisms 

 that may be present on such a surface. Tetanus, or lock- 

 jaw, is a striking illustration of this method of infection. 



