206 BACTERIOLOGY. 



the operator and the source of light. This renders 

 visible by transmitted light not only the coarser vessels 

 of the ear, but also their finer branches. The point at 

 which the injection is to be made is to be shaved clean 

 of hair, by means of a razor and soap. 



The filled hypodermic syringe is taken in one hand 

 and with the other hand the ear is held firmly. The 

 point of the needle is then inserted through the skin 

 and into the finest part of the ramus posteriory the part 

 nearest the apex of the ear, where the course of the 

 vessel is nearly straight. When the point of the 

 needle is in this vessel it gives to the hand a sensation 

 quite different from that felt when it is in the midst of 

 connective tissue. As soon as one thinks the point of 

 the needle is in the vessel, a drop or two of the fluid 

 may be injected from the syringe, and if his suspicions 

 are correct the circulation in the small ramifications and 

 their anastomoses will quickly alter in appearance. 

 Instead of their containing blood, the colorless fluid 

 which is being injected will now be seen to circulate. 

 This must be carefully observed, for sometimes when the 

 needle-point is not actually in the vessel, but is in the 

 lymph-spaces surrounding it, an appearance somewhat 

 similar is to be seen. It may always be differentiated, 

 however, by continuing the injection, when the circu- 

 lation of clear fluid through the vessels will not only 

 fail to take the place of the circulating blood, but 

 there will at the same time appear a localized swelling 

 under the skin about the point of the needle. The 

 needle must then be withdrawn and inserted into the 

 vessel at a point a little nearer to its proximal end. 



Care must be taken that no air is injected. 



The hypodermic syringe and needle must, previous 



