318 



TRKPTTTNTNfi TTTTC FACIAL STNUSTCR. 



the membrane separated with a scraper or blunt knife. A central 

 hole having been made with a gimlet, the trephine, with its centre 

 pin in advance, is replaced in position, and by light rotary 



movements caused to enter 

 the bone. The instrument 

 in case of need is supported 

 by the thumb and index 

 finger of the operator's left 

 hand. The teeth of the saw 

 soon make a groove, and 

 when once the trephine has 

 obtained a "grip" it should 

 be turned in one direction 

 until the bone is divided. 

 Where the bone is very 

 thick it will be necessary 

 from time to time to clean 

 the teeth of the trephine 

 with a brush. An increased 

 sense of yielding gradually 

 becomes apparent, where- 

 upon sawing is more 

 cautiously proceeded with. 

 As soon as the bone is 

 cut through, the elevator is 

 applied and the piece raised 

 and lifted out. Frequently 

 the disc of bone will be 

 found fixed in the trephine 

 and removed with it. The 

 lining mucous membrane 

 generally remains in situ, 

 or hanging in shreds to the 

 sides of the opening. 



After removing sharp 

 edges with the lenticular 

 knife, the finger or a probe 

 may be introduced for exam- 

 ination. Bleeding is usually 

 slight. In empyema, pus usually flows from the opening spon- 

 taneously, and whatever remains can be removed by washing. After- 

 wards the opening is closed with a plug of tow or gauze. 



Fig. 316. — Head of a horse. 

 boundaries of the frontal sinus. 

 ,, ,, nasal meati. 



,, ,, superior maxillary sinus. 



