214 BACTERIOLOGY. 



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 mate- 

 rial 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 tendency to pene- 

 trate through the opposite wall. We never use a new 

 hypodermic 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. 



Fig. 46. 

 a 



Hypodermic needles magnified, a. Improper point. 6. Proper shape of point. 



In Fig. 46, a, the needle has the point usually seen 

 when new. 



In Fig. 46, b, 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. 



