INJECTION INTO THE CIRCULATION. 217 



into the loose tissue under the skin over this part of the 

 back in the way that has just been described. It is 

 always best to insert the needle some distance along the 

 spinal column, and thus deposit the material as far from 

 the surface-wound as possible. 



INJECTION INTO THE CIRCULATION. It is not in- 

 frequently desirable to inject the material under consid- 

 eration directly into the circulation of an animal. If 

 a rabbit is employed for the purpose, the operation is 

 usually done upon one of the veins in the ear. To those 

 who have had no practice with this procedure it offers a 

 great many difficulties ; but if the directions which will 

 be given are strictly observed, the greatest of these 

 obstacles to the successful performance of the operation 

 may be overcome. 



When viewing the circulation in the ear of the rabbit 

 by transmitted light three conspicuous branches of the 

 main vessel (vena auricularis posterior) will be seen. 

 One runs about centrally in the long axis of the ear, 

 one runs along its anterior margin, and one along its 

 posterior margin. The central branch (ramus anterior 

 of the vena auricularis posterior) is the largest and most 

 conspicuous vessel of the ear, and is, therefore, believed 

 by the inexperienced to be the branch into which it would 

 appear easiest to insert a hypodermic needle. This, 

 however, is fallacious. This vessel lies very loosely 

 imbedded in connective tissue, and, in efforts to intro- 

 duce a needle into it, rolls about to such an extent that 

 only after a great deal of difficulty does the experiment 

 succeed. On the other hand, the posterior branch (ramus 

 later alls posterior of the vena auricularis posterior) is a 

 very fine, delicate vessel which runs along the posterior 



