THE SINUSES OF THE SKULL 



95 



long axis of which is about two inches in extent and is directed 

 vertically. This sinus may be said to have two compartments^ which 

 are incompletely separated from one another by a ridge of bone which 

 bounds the superior dental canal ; over this ridge there is free 

 communication between the two divisions. 



The inner compartment, which is the smaller of the two, extends 

 beneath the floor of the orbit, and superiorly it is in communication 

 with the sphenoidal sinus. This is the division which also communi- 

 cates with the middle meatus of the nose, by a curved slit-like 

 aperture near the upper part of the latter. 



The outer division extends in the upward direction into the maxillary 

 protuberance, at the upper extremity of the superior maxilla behind the 

 maxillary hiatus. Its lower limit extends to a level which is situate 

 from one and a half to two inches above the lower extremity of the 

 zygomatic ridge. The lower extremity of this compartment is separated 

 from the inferior maxillary sinus by an imperforate, horizontally placed, 

 delicate, osseous septum. The position of this plate is extremely 

 important, and will be again referred to in describing the operation of 

 trephining into the sinus. The degree of development of the septum 

 is also said to be variable. According to Lanfiletti, it attained a high 

 degree of development in about 75 per cent, of a large number of 

 cases examined. 



The root of the sixth (and in part also that of the fifth) upper molar 

 tooth projects into the posterior wall of this sinus, where it is covered by 

 a thin plate of bone which forms part of the boundary of the alveolus. 

 The bulgings formed by this plate of bone in the young animal almost 

 entirely occupy the cavity of the outer compartment, but as the animal 

 advances in age the teeth are pushed out from their sockets, the alveoli 

 diminish in size, and a corresponding enlargement of the sinus takes 

 place. 



In addition to the superior maxilla this sinus is also bounded by part 

 of the lachrymal bone, the malar, the posterior tubinated bone, and 



