TREPHINING. 



287 



The hone-scrapers, or elevators, whicli are sometimes employed 

 as accessory, are used for preserving the periosteal covering of 

 the bones; for removing the rough edges of the opening; or in 



^^P 



Fig. 302.— Bone Scrapers and Elevators. 



raising the bony fragments vi^hich have been crushed in, or may 

 be pressing against the cerebral substances. 



An important step before entering uj^on the operation, is to 

 determine the points which are to be avoided. Generally speak- 

 ing, any part of the head or of the body can be oj)erated upon, so 

 long as there is a bony sui-f ace that can be readily exposed without 

 danger of injuring other important organs. But, upon the head, 

 the angles of bones and the tl-acts of the cranial sutui-es ought to 

 be avoided. In the more common apphcations of the operation 

 upon the head (Figs. 303, 304), one of four princij^al j^oints is gen- 

 erally selected, by which to effect an entrance into the cavities of 

 the sinuses. The points marked h, near the lower borders of the 

 frontal bone, will open the frontal sinuses : the point c, upon the 

 surface of the nasal bones, communicate with the sujDerior part of 

 the nasal canities in the upper portion of the turbinated bone; the 

 points e, upon the zygomatic bone, will enter the superior, and /', 

 upon the great maxillary bone, will penetrate the lower maxillary 

 sinus. Besides these specific regions upon which to apply the 



