98 EXPERIMENTS UPON ANIMALS. 



Generally, in making injections into animals, it is customary to 

 remove the hair for some distance around the point of inoculation 

 with scissors and razor, and then to sterilize the surface by careful 

 washing with a solution of bichloride of mercury. This precaution 

 is necessary in researches in which pathogenic bacteria are being 

 tested, in order to remove any possibility of accidental inoculation 

 with germs other than those under investigation, and, as a conse- 

 quence, a mistaken inference as to the pathogenic action of the spe- 

 cies under investigation. But when we know the specific pathogenic 

 power of a certain microorganism it is hardly necessary to take this 

 precaution, as a few drops of culture will contain millions of the bac- 

 teria, while contamination, if it occurs from the surface of the body, 

 must be by a comparatively small number of bacteria, which are 

 likely to be of a harmless kind which will have no influence on the 

 result of the experiment. 



Instead of sterilizing the surface, the writer usually clips away a 

 small portion of skin with curved scissors, not cutting deep enough 

 to draw blood, but leaving a bare surface through which the point of 

 the syringe can be introduced with very little danger of carrying bac- 

 teria into the connective tissue other than those contained in the 

 syringe. * 



In making injections into the peritoneal cavity care must be taken 

 not to wound the liver or the distended stomach. The intestine is 

 not very likely to be wounded, as it slips out of the way. By seizing 

 a longitudinal fold of the abdominal wall and pushing the point of 

 the syringe quite through it, and then releasing the fold and care- 

 fully withdrawing the instrument until the point remains in the 

 cavity, the danger of wounding the intestine will be reduced to a 

 minimum. 



Injections into the circulation are made by exposing a vein and 

 carefully introducing the needle of the syringe in the direction of 

 the blood current. Care must of course be taken not to inject air. 

 In the rabbit one of the large veins of the ear may be conveniently 

 penetrated by the point of a hypodermic syringe without any pre- 

 vious dissection. The ear is first washed with a solution of bichloride 

 of mercury or simply with warm water. The animal had better be 

 carefully wrapped in a towel to control its movements. The veins 

 are distended by compressing them near the base of the ear. When 

 the point of the needle has not been properly introduced, and the 

 fluid to be injected escapes in the surrounding connective tissue, it 

 will commonly be best to withdraw the syringe and make the 

 attempt upon another vein. As pointed out by Abbott, the needle 

 of the syringe should be ground flat at the point, and not curved as 

 is commonly the case. 



