POSITION FOR TREPHINING. 
G9 
the periosteum. The superior maxillary sinus in the horse is generally 
divided by a thin plate of bone into an upper larger and a lower smaller 
division. Lanzilotti describes this plate as having been present m 52 
out of 74 cases operated on. It lies nearly in the centre of a rectangle, 
whose longer sides are formed by the zygomatic ridge, and a line drawn 
parallel with it, starting from the inner angle of the eye, and whose 
shorter sides are bounded by the rim of the orbit, and a line drawn 
perpendicularly to the lower end of the zygomatic process of the malai 
bone (see figs. 48 and 49). According to Lanzilotti, the septum is always 
wanting in the ass, and often in the mule. Trephining about 1J inches 
above the lower end of the zygomatic ridge, and f to 1\ inches away fiom 
Fig. 42.— Fig. 43.— 
Bone elevator Chisel, 
or screw. 
Fig. 44.— Fig. 45.— 
Lenticular Periosteum 
knife. knife. 
Fig. 46.— Fig. 47.— 
Exfoliative Perforating 
trephine. trephine. 
it, this division is cut into, and both portions of the sinus are opened. 
Where one sinus alone has been opened, the dividing wall can be broken 
down. In foals one operates somewhat further from the edge of the 
zygomatic ridge, in order not to injure the roots of the fourth and fifth 
molars which lie in the lower division of the superior maxillary sinus. 
The horse had better be cast, though quiet animals may be operated 
on standing. After removing the hair from a space about 2 to 2| inches 
in circumference, a longitudinal cut 2 to 3 inches m length is made 
through the skin. This should be parallel with the middle line, and 
| to 1} inches removed from it. 
In trephining the superior maxillary sinus, the cut should be parallel 
with, and about 1 inch above, the zygomatic ridge. A “ T ” or “ V ” 
shaped incision was formerly recommended, but Gerlach has rightly 
condemned this procedure, because the loose flaps of skin retract and 
