POSITION FOR TREPHINING. 



317 



horses one operates somewhat further from the edge of the zygomatic 

 ridge, in order not to injure the roots of the 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 in the standing position. The hair should be removed 

 from the seat of operation, which is then washed and disinfected. 

 Anaesthesia is not necessary. 



In trephining the superior maxillary sinus, the incision should 

 be parallel with, and about 1 inch above, the zygomatic ridge. A 

 " T " or " V " or " X " shaped incision was formerly recommended, 



but Gerlach rightly condemned this procedure, because the loose 

 flaps of skin retract and give rise to excessive granulation. The 

 late Professor Robertson recommended removing a circular patch 

 of skin. Cicatrisation is much more perfect after this procedure, 

 and in one or two months no visible wound remains. The skin is 

 separated from the sub-lying tissues with the knife in order to make 

 room for the trephine crown within the edges of the wound. Should 

 the operation on the superior maxilla be performed rather higher 

 than above described, the belly of the levator labii superioris is 

 encountered, and must be pushed to one side. The trephine crown 

 is now placed in position, and to facilitate removal of the pieces of 

 periosteum a cut is made around it with a guarded bistoury, and 



