INJECTION INTO THE CIRCULATION. 215 



When the operation is to be performed an assistant 

 holds the animal gently bat firmly in the crouching 

 position upon a table. If the animal does not remain 

 quiet, it is best to wrap it in a towel, so that nothing but 

 its head protrudes ; though in most cases we have not 

 found this necessary, and particularly if the animal has 

 not been excited prior to the beginning of the operation. 



The animal should be placed so that the ear upon 

 which the operation is to be performed comes between 

 the operator and the source of light. This renders vis- 

 ible 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 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 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 some- 

 what similar is to be seen. It may always be differen- 



