68 
TREPHINING INSTRUMENTS. 
held. The other end is fashioned into a conical screw, which can be 
inserted in the piece of loose bone to remove it: under certain circum¬ 
stances the elevator may be used to bring dislocated fragments of bone 
into their former position. 
(3.) I he lenticular knife (fig. 44). With this the sharp edges of the 
opening left by the trephine are removed, to prevent the finger being 
injured when introduced during examination. 
(4.) The periosteum knife ” (fig. 45) is for scraping the periosteum 
from the point of operation, but may be replaced by a blunt knife, 
Fig. 40.— Stock trephine. 
Fig. 41.—Hand trephine. 
curette or gouge. The set of instruments usually comprises a chisel 
(fig. 43) to remove pieces of bone such as those between two trephine 
apertures, and is useful in making large openings. Where a trephine is 
not to hand, the opening may be made with an ordinary borer (exfolia¬ 
tive trephine, fig. 46), but if it is merely necessary to make an aperture 
in the bone, the perforating trephine is used (fig. 47). The last-named 
instruments can, however, generally be dispensed with. 
Trephining the facial and superior maxillary sinuses of the horse. 
In opening the facial sinus, the frontal bone is trephined at a point 
on a level with the supra-orbital process of the frontal bone, and about 
1 inch fiom the middle line. Here the skin lies almost immediately on 
