INJECTION INTO THE CIRCULATION. 219 



presenting a flat, slanting surface when viewed from the 

 side, has a more or less curved surface. Now, in efforts 

 to introduce such a needle into a vessel of very small 

 calibre it is usually seen that the point of the needle, 

 instead of remaining in the vessel, as it would do were 

 it straight (or "chisel pointed"), very commonly pro- 

 jects into the opposite wall ; and as the needle is inserted 

 further and further it is usually pushed through the 

 vessel-walls into the loose tissues beyond, and the 

 material to be injected is deposited in these tissues, 

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

 the slanting point of the needle be ground until its sur- 

 face is perfectly flat when viewed from the side, and no 

 curvature exists, then when once inserted it usually 

 remains within the vessel, and there is no tendency to 

 penetrate the opposite wall. We never use a new hypo- 

 dermic needle until its point is carefully ground to a per- 

 fectly flat, slanting surface with no curvature whatever. 

 These differences may perhaps be more easily under- 

 stood if represented diagrammatically. In Fig. 44, a, 



FIG. 44. 

 a 



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



the needle has the point usually seen when new. In Fig. 

 44, b, the point has been ground 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 



