THE SINUSES OF THE SKULL 107 



and it therefore always requires a separate opening for the evacuation 

 of its contents. 



Positive diagnosis of the affection in connection with this sinus 

 should be made by exploration with a gimlet. 



In describing the outline of this sinus it will be remembered that 

 the line indicating its inferior boundary was most irregular, and that 

 at some points it did not extend to a lower level than the inferior 

 extremity of the zygomatic ridge, though the lowest limit of the 

 sinus was about three-quarters of an inch inferiorly to this point. 



The inferior limit of our trephined opening must therefore not 

 come below the inferior extremity of the ridge, otherwise there would 

 be a probability that our opening may not communicate with the 

 cavity of the sinus. The area is shaved and cleansed as before. The 

 flap of skin having been reflected, the bone is found to be sub- 

 cutaneous. Care is necessary in this case to avoid the angular artery 

 and vein, and particularly when operating on the young animal when 

 a seat somewhat more anteriorly placed is selected as in the opera- 

 tion on the superior maxillary sinus, in order to avoid injury to the 

 walls of the alveoli of the molar teeth. When the superior and 

 inferior maxillary sinuses are affected the contents of both may be 

 expelled through the opening just described, by breaking down the 

 septum which separates the two sinuses. This is a method which is 

 frequently adopted. 



