14 VETERINARY DENTAL SURGERT. 



large and divided into two chambers by a transverse 

 plate of bone which always remains perfect through 

 life, completely isolating the anterior chamber. This 

 plate of bone is usually opposite the space between the 

 fourth and fifth molar teeth, so that by trephining over 

 that region an opening will be made in both compart- 

 ments. The posterior of these chambers, sometimes 

 known as the superior maxillary sinus, is continued 

 backwards to the alveolar tuberosity and contains the 

 roots of the last two molar teeth. The anterior 

 division, sometimes called the inferior maxillary 

 sinus is the smaller of the two and has the roots of 

 the fourth, occasionally the third molar teeth pro- 

 jecting into it. 



The sphenoidal sinus is small and formed by the 

 sphenoid and palatine bones. It is subdivided by 

 incomplete partitions into several compartments. 



The ethmoidal sinus the smaller of these cavities 

 is a space included in the ethmoid bone. It com- 

 municates with the maxillary sinus. 



