98 THE SURGICAL ANATOMY OF THE HORSE 



superior, has two divisions, but these are in communication with one 

 another over the osseous ridge which is the continuation ot the superior 

 dental canal. This sinus is placed in the superior maxillary bone. The 

 inner division of the sinus is the smaller, and it communicates directly 

 with the middle meatus of the nose. It is bounded inwardly by the 

 posterior tubinated bone. The outer division is situate directly beneath 

 the corresponding division of the superior maxillary sinus, from which, 

 as already stated, it is completely separated by a thin and imperforate 

 septum. The upper limit of this sinus therefore would be indicated by 

 a line drawn horizontally in a position which is from one and a half to 

 two inches above the lower extremity of the zygomatic ridge. The 

 lower limit of the sinus may be likewise indicated by a line drawn 

 horizontally about three-quarters of an inch below the inferior extremity 

 of the zygomatic ridge. This line, however, takes a very irregular 

 course, and on this account an opening made below the inferior 

 extremity of the ridge may not necessarily lead into the sinus. 



The tracing of this sinus on the exterior of the skull may be made as 

 follows : 



Its superior boundary would be represented by the line already 

 drawn as indicating the inferior boundary of the superior maxillary 

 sinus. Its anterior boundary is indicated by the inferior portion of the 

 line representing the course of the lachrymal duct, its posterior boundary 

 may be represented by a continuation of the line which indicates the 

 posterior boundary of the superior maxillary sinus, and its inferior 

 boundary by the irregular line already referred to. 



The Sphenoidal Sinuses. — The sphenoidal sinuses are the smallest of the 

 sinuses of the skull. They are also of least importance from a surgical 

 point of view, since they are deeply seated, near the base of the cranium, 

 and are consequently in an inoperable position. Each sinus is divided into 

 two portions, namely, sphenoidal and palatine. The sphenoidal division 

 is an excavation in the anterior or lower portion of the body of the 

 pre-sphenoidal bone. This portion is divided up into a number of 



