42 THE SURGICAL ANATOMY OF THE HORSE 



Whilst working in the sinus, it will be observed that we are also in 

 close proximity to the osseous superior dental canal, and this should be 

 avoided, particularly during the use of the chisel, otherwise injury to the 

 important structures running though the canal, which include the 

 sensory superior dental portion of the superior maxillary division of 

 the fifth nerve, is a very liable complication. 



The root of the fifth upper molar takes a direction which more 

 nearly approaches the vertical than does that of the sixth. In this case 

 the centre of the trephine is fixed at a point which is 1.25 inch 

 anteriorly to that selected in the previous case, and an inch above or 

 in front of the zygomatic ridge. 



The root of the fourth upper molar, as already mentioned, forms a 

 prominence which is entirely in the inferior maxillary sinus, and the 

 piece of bone removed is similar to that removed for the evacuation 

 of the contents of the sinus. {See Facial Sinuses.) 



The anterior limit of the inferior maxillary sinus would be indicated 

 by a vertical line drawn three-quarters of an inch in front of the 

 extremity of the zygomatic ridge. Now the root of the third molar is 

 indicated by an elevation which for the greater part is visible in 

 this sinus. 



If therefore the centre of the trephined opening be taken at a point 

 which is half an inch in front of the extremity of the ridge, and 

 1.5 inch above it, the opening will lie over the extremity of the 

 root. The openings in this and the next case are of great importance 

 in order that we may keep clear of the infra-orbital nerve, where it 

 appears on the face through the foramen of the same name. 



The circular opening for the removal of the second molar should 

 be an inch further forwards, but in line with that made in the removal 

 of the third, whilst that for the first molar should, on account of the 

 great antero-posterior dimension of this tooth, and the compartively short 

 root which it possesses, be made one inch and a half anteriorly to that 

 for the second molar, and half an inch nearer the tables of the teeth. 



