18 TREPHINING OF THE FRONTAL SINUSES 



probe pointed bistoury, dressing forceps, disinfecting and 

 dressing materials. 



Technic. The operation may be performed upon the 

 standing animal with the aid of local anaesthesia of the 

 skin, the bone having virtually no sensation. Restless 

 animals may be further secured with the twitch, in the 

 stocks, upon the operating table or by casting on the sound 

 side. 



Clip and shave the hair from the region of the frontal 

 bone at that point which the operator has reason to believe 

 is nearest the center of disease. The highest point at which 

 the sinus may be trephined is indicated by F in Fig. 3. At 

 this point the sinus is very shallow, its floor consisting of 

 the cranial plate of the frontal bone. The most central 

 portion of the cavity is reached by trephining on a level 

 with the inferior border of the orbital cavity on the Hues 

 FE, Figs. 5 and 6. Trephining at this point gives the 

 operator access to the superior maxillary sinus, SM, Figs. 

 4-6, through the fenestrum, FE, Figs. 5 and 6. The 

 lowest and generally most essential point for trephining is 

 at ST, Figs. 4 and 5, where the opening affords free drain- 

 age externally from the most dependent part of the cavity 

 and at the same time offers ample opportunity for securing 

 dependent nasal drainage by breaking through the superior 

 turbinated bone at ST, Figs. 4, 5 and 11. 



By consulting Figs. 7-9, it will be seen that after reach- 

 ing the level of the nasal septum, a trephine opening im- 

 mediately against the median line like that at F, Fig. 3, 

 would wound the septum and superior turbinated bone and 

 penetrate the nasal cavity. Consequently the operator 

 must avoid making the trephine opening in this region near 

 the median line, but must keep i^ to 2 inches laterally 

 therefrom. 



With a heavy convex scalpel, make a circular incision at 

 the desired point as large as the area of the trephine, 



