I04 THE SURGICAL ANATOMY OF THE HORSE 



to Plate III., we are reminded that the greatest antero-posterior 

 dimension of the sinus is about its middle, and that this dimension 

 progressively diminishes as we proceed from a transverse plane indi- 

 cating the middle of the sinus, either in the upward or downward 

 direction. 



To adopt a seat too high up the forehead would render injury to the 

 brain a liable complication, whilst to adopt a seat which is correspond- 

 ingly too low down would bring about the liability to injury to the 

 ethmoidal lateral mass. Again referring to the Plate we observe 

 that if a horizontal line be drawn from one supra-orbital foramen to 

 the other, it would indicate the upper limit of an area which from 

 an operating point of view may be termed sate, since almost immediately 

 below such a line the sinus presents its greatest depth, whilst above 

 the line its antero-posterior dimension decreases somewhat rapidly. 



The supra-orbital foramina should therefore be located, and an 

 imaginary horizontal line drawn across from one to the other. (Some 

 operators chalk, out the line.) A short distance below this line then 

 must be the upper limit of the opening to be made. The line thus 

 drawn should now be bisected by a line which runs vertically, namely, at 

 right angles to the first line. This second line gives us the median 

 sagittal plane, and, as before remarked, it will indicate the position 

 of a plate of bone which forms the partition between the two 

 frontal sinuses. Care must therefore be taken to avoid this line 

 in operating, in order not to open down on to the septum. 



In order lo effect this the method of procedure should be as 

 follows : 



To bisect each half of the originally bisected horizontal 

 line, when we now arrive at a point of bisection exactly 

 below which the centre of our trephined opening should be 

 placed, and half an inch below which point the circum- 

 ference of the circular opening should pass. 



According to MoUer, " the frontal sinus is trephined at a point 



