190 BACTERIOLOGY. 



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 is ground down until its 

 surface is perfectly flat, and 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 ten- 

 dency to penetrate through the opposite wall. We never 

 use a new hypodermatic needle until its point is carefully 

 ground down to a perfectly flat, slanting surface and no 

 more curvature exists. 



These differences may perhaps come out clearer if 

 represented diagrammatically. 



Pig. 37. 



a 



Hypodermatic needles, a, Improper point ; 6, proper shape of point. 



In Fig. 37, a, the needle has the point usually seen 

 when new. 



In Fig. 37, 6, the point has been ground down to the 

 shape best suited for this operation. 



The needles need not be returned to the maker. One 

 can grind them to the shape desired in a few minutes 

 upon an oilstone. 



The size of the needle is that commonly employed for 

 subcutaneous injections. 



When the operation is to be performed, an assistant 



