96 THE SURGICAL ANATOMY OF THE HORSE 



the lateral mass of the ethmoid. The boundaries of the outer wall 

 of the sinus may be traced out on a skull as follows : 



The superior border would be indicated by a line drawn 

 transversely from the zygomatic ridge forwards to the nasal canthus. 

 The anterior boundary may be similarly indicated by a line drawn 

 obliquely downwards and forwards from the nasal canthus for a length 

 of one and a half to two inches in the direction of the nasal peak. The 

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

 along the zygomatic ridge, commencing at a point from one and a half 

 to two inches above its lower extremity, and terminating superiorly 

 where it intersects the line already drawn as indicating the superior 

 limit of the sinus. A line connecting the inferior extremities of the 

 lines indicating the anterior and posterior limits, would represent the 

 position of the septum separating the superior and inferior maxillary 

 sinuses. 



The Frontal Sinus. — In size this sinus is the second largest, and it is 

 placed towards the front of the face. It extends in the upward 

 direction between the reflected plate and the body of the frontal bone, 

 and this part of it is placed directly in front of the cranial cavity. 

 On account of the disposition of the reflected plate and the body of 

 the frontal bone to one another, it will be gathered that the size of the 

 sinus from before backwards, diminishes as we ascend to the position 

 where the reflected plate is united to the body. This point should be 

 noted in connection with the operation of trephining into this sinus, 

 for should the error be committed of adopting a seat too high up 

 the forehead, there is an evident danger of opening into the cranial 

 cavity, and causing serious injury to its contents. 



The right and left frontal sinuses are separated from one another 

 in the median line by a complete bony septum, and there is, therefore, 

 no direct communication between them. This septum forms the 

 greater part of the inner boundary of the sinus. It extends vertically 

 for about four inches, and then takes a course which is slightly 



