tmncTION INTO TUE OIRCULATtON. 239 



found this necessary, particularly if the animal has not 

 been excited prior to beginning the operation. 



Ths ear in which the injection is to be made should be 

 shaved clean of hair by means of a razor and soap and 

 then washed with water. It is unnecessary to attempt 

 disinfection of the skin. 



The animal should be placed so that the prepared 

 ear comes between 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 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 posterior, 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 con- 

 nective tissue. As soon as one supposes 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 rapidly alter in appearance — 

 i. c, the circulating blood will be displaced very quickly 

 by the clear, transparent fluid that is being injected. At 

 this stage one must proceed very carefully, for some- 

 times when the needle-point is not actually in the ves- 

 sel, but is in the lymph-spaces surrounding it, an ap- 

 pearance somewhat similar is seen. This may always 

 be differentiated, however, by continuing the injection, 

 when the flow of clear fluid through the vessels will not 

 only fail to take the place of the circulating blood, but 

 at the same time a localized swelling, due to an accu- 



