204 BACTERIOLOGY. 



in the dense tissues which surround it that it is pre- 

 vented from rolling 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. 



Select, then, the very delicate vessel lying quite close 

 to the posterior margin of the ear, and make the injec- 

 tion as near to the apex of the ear as possible. 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 way in which their points are ground. If one 

 examines 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, is more or less of a curved surface. Now, in 

 efforts to introduce such a needle into a vessel of very 

 small calibre, it is commonly seen that the extreme 

 point of the needle, instead of remaining in the vessel, 

 as it would do were it straight, very commonly projects 

 into the opposite wall, and as the needle is inserted 

 further and further into the tissues, it is usually pushed 

 through the vessels into the loose tissues beyond, and 

 the material to be injected is deposited into these tissues 

 instead of into the circulation. If, on the contrary, the 

 slanting point of the needle be ground down until its 

 surface is perfectly flat when viewed from the side, 

 and no more curvature exists, then when once inserted 

 into a vessel it usually remains there, and there is no 



