TKEPHINING. 



289 



Fig. 305.— Common Points of Selection for Trephining. 



semi-lunar incision. We prefer the first as being least liable to be 

 followed by a blemish. The periosteum is divided in the same 

 form, if its division has not ah'eady followed that of the skin. 

 The flap thus formed in two structures is then carefully raised 

 from the attachment to the external surface of the bone, and 

 held aside by an assistant, with either a blunt tenaculum or 

 forceps. The instrument is then implanted upon the centre of 

 the exfiosed bony surface, and by the rotatory or semi-rotatory 

 motion imparted to it, gradually separates a circular disk of the 

 bone. The pressure necessary to make the instrument seize or 

 bite on the bone may at first be considerable, but as the trephine 

 penetrates, the force" must be carefully relaxed in order to avoid 

 the hazard of injuring the j^arts beneath by the sudden yielding 

 of the bone and plunging of the instrument into the parts beneath, 

 accompanied by the bony disk, as it separates from its last attach- 

 ments, perhaps passing beyond reach, and costing no Uttle touble 

 and danger before it can be discovered and removed. 



It is not judicious to penetrate through the bone at once. As 

 the operator feels that he has nearly reached the last turn, the 



