THE SINUSES OF THE SKULL loi 



already mapped out as indicating the position of the sinuses. In the 

 place of this sound we now have a dull thud. 



At times the area over the frontal bone loses its flattened appear- 

 ance and becomes convex on account of the bulging forv/ards of the 

 underlying bone. When the affection has reached an advanced stage, 

 the bones become softened, and occasionally the finger, without utilising 

 a great amount of force, may be pressed through the bone into the 

 cavity of the sinus. Positive diagnosis may be made by exploration 

 with a gimlet. 



After diagnosis of the condition the operation of trephining into 

 the sinuses becomes necessary in order that these cavities may be 

 emptied of their purulent contents, and also in order that they may 

 afterwards be injected out with antiseptic solutions. 



In cases of tumours or foreign bodies, such as fractured pieces of 

 bone in the sinuses, the seat of operation must be as nearly as possible 

 over the area affected. In such cases it is advisable to revise the 

 general superficial anatomy of the areas of the sinuses. {See Plate XIV., 

 Superficial Dissection of Face.) 



The following description refers to the performance of the operation 

 in the treatment of empyema. 



Trephining into the Superior Maxillary Sinus. — It is advisable in 

 performing the operation of trephining into this sinus, to make the 

 trephined opening in such a position that the lower portion of its 

 circumference will be on as near as possible an exact level with the 

 osseous septum separating the superior and inferior maxillary sinuses. 

 The object in selecting this position is in order that the septum 

 referred to shall form a natural floor for drainage, and also that 

 there shall not be " pocketing " of purulent material behind the 

 trephined opening. 



The usual position of this septum in an average-sized animal is 

 one inch and a half above the inferior extremity of the zygomatic 

 ridge of the superior maxillary bone. The distance of the septum 



