DISEASES OF THE FACIAL SINUSES. 



519 



nose, as the fangs of the molar teeth are then long, and their 

 alveoli fill up the sinus, whereas in the old horse the sinus will 



Fig. 110. — Skull cut open to show the situation of bony partition of 

 superior maxillary sinus. The white circle is the spot selected for tre- 

 phining both cavities by one opening, a. Space between fourth and fifth 

 molar teeth ; h, Termination of zygomatic spine. 



extend as low as the zygomatic crest, where the opening can be 

 made without danger of coming in contact with the teeth. 



In some instances the frontal sinus has to be opened, especially 

 when the contained pus is inspissated, and lodged in the convolu- 

 tions of the ethmoidal and turbinated bones ; but in the majority 

 of instances one opening, at the most depending part of the cavity, 

 is sufficient. 



The external portion of the maxillary sinus is divided into 

 two compartments by a transverse plate, marked in the drawing, 

 which in many cases is perfect throughout life. The superior 

 portion contains the alveoli of the two last molar teeth, the 

 inferior portion the alveoli of the third and fourth, while the 

 first and second molars are quite superficially situated; very 

 often, indeed, their fangs actually protrude through the maxil- 

 lary bone, or they may be so thinly covered by it that they can 

 be both seen and felt through the skin of a well-bred horse. 

 From this short description it will be seen that the guide to the 

 operator in making the opening through the bone, so as to em- 

 brace both cavities, will be the space between the fourth and fifth 

 molar teeth. Of this he must judge by an inspection of the 



