METHODS OF INFECTION. 269 



4. Intra-peritoneal injection.. This method of experi- 

 mental infection is employed as frequently as the subcu- 

 taneous one already described. When organisms are 

 introduced directly into the peritoneal cavity, the results 

 of infection rapidly follow. The animal may die in 12 

 hours or less. Moreover, it should be noted that many 

 bacteria, which will not kill when injected subcutaneously, 

 are invariably fatal when introduced into the peritoneal 

 cavity. A good example of this is seen in the colon 

 bacillus. 



The animal to be injected is placed on its back, or, in 

 the case of the mouse or rat, it is suspended in the manner 

 described on p. 265. A fold of the skin is raised and the 

 needle introduced at an angle of about 45 degrees. The 

 fact that the needle enters the peritoneal cavity is recog- 

 nized by the absence of resistance to the needle and by the 

 absence, at the point of inoculation, of a local swelling 

 which would indicate that the fluid has entered the subcu- 

 taneous tissue. As a rule, there is very little or no danger 

 of injuring the intestines. In making injections it is advis- 

 able to cut away the hair over the point of inoculation, 

 thus marking as it were, the place of operation. . It is 

 hardly necessary, as a rule, to attempt a thorough disin- 

 fection of the skin previous to making an injection. 



Intra-peritoneal injection into a large animal, as the 

 horse, is made by means of a trochar. The animal is held 

 in the standing position and the trochar introduced through 

 the abdominal wall, after previously cutting the skin 

 with a lance. A point 4 or 5 inches in advance of the 

 anterior iliac spine is selected as the site of inoculation. 



In certain cases it is desirable to observe the disease 

 process in the peritoneal cavity, or to introduce solid 

 masses into this cavity. In that event it is necessary to 

 make a laparotomy, in which case the utmost precaution 

 to prevent accidental infection must be observed. This 

 operation is resorted to whenever the collodium sac method 



