222 BACTERIOLOGY 



about under the point of the needle. The further away from 

 the mouth of the vessel — that is, the nearer we approach 

 its capillary extremity — the more favorable become the 

 conditions for the success of the operation. 



After shaving the ear and carefully washing it with clean 

 water select the very delicate vessel lying quite close to the 

 posterior margin of the ear, and make the injection as near 

 to the apex of the ear as possible. At times the vessels of 

 the ear will be found to contain so little blood that they are 

 hardly distinguishable, making it very diflScult to introduce 

 the needle into them. This is sometimes overcome by pres- 

 sure at the root of the ear, causing stasis of the blood and 

 distention of the vessels. A very satisfactory method of 

 causing the veins to become prominent is to press lightly or 

 prick gently with the point of a needle the skin over the 

 vessel to be used. In a few seconds, as a result of this irri- 

 tation, the vessel will have become distended with blood, 

 and readily distinguishable from the surrounding tissue; 

 it may then be easily punctured by the needle of the syringe. 

 A sharp flick with the finger will often produce the same 

 result. The injection is always to be made from the dorsal 

 surface of the ear. 



Of no less importance than the selection of the proper 

 vessel is the shape of the point of the needle employed. 

 The hypodermic needles as they come from the makers 

 are not suited at all for this operation, because of the manner 

 in which their points are ground. If one examine carefully 

 the point of a new hypodermic needle, it will be seen that 

 the long point, instead of presenting a flat, slanting surface 

 when viewed from the side, has a more or less nirved surface. 

 Now, in efforts to introduce such a needle into a vessel of 

 very small calibre it is usually seen that the point of the 



