THE SINUSES OF THE SKULL 103 



forwards a short distance in front of the seat. Greater care, however, 

 is necessary when operating further forwards, as in the young 

 animal. 



A similar incision is next made through the levator labii 

 superioris alasque nasi. Little if any hemorrhage results from such 

 an incision, since the aponeurosis is so thin. But no incision must 

 be made into the proprius muscle. It is unnecessary to incise 

 it, for since it is quite loosely applied to the anterior aspect of the 

 zygomatic ridge, it is easily displaced and pushed forwards out of 

 the way of the operator. This should be done, and thus the 

 considerable hemorrhage which would necessarily follow an incision 

 into the muscle is avoided. 



The bone covered by its periosteum is now exposed, and the 

 periosteal layer is removed by means of a periosteotomy knife. 



With a gimlet a hole should now be bored in the centre of 

 the exposed area. This hole is for the accommodation of the central 

 spike of the trephine. 



In removing the circular piece of bone, the saw should be 

 worked at first to and fro towards the right and left alternately, until 

 a hold is obtained, when the movement should be in one direction 

 only. This part of the operation is facilitated by applying a greater 

 amount of pressure to one side of the saw than to the other, and 

 this side of the bone will therefore be pierced first. When this 

 occurs the circular piece of bone may be usually broken off, and 

 care must be taken that it does not fall into the sinus. This may 

 be effectively done by snapping off the piece or bone to be removed 

 by means of a rigid probe inserted into the central hole first 

 made, just before section with the saw is complete. 



Trephining into the Frontal Sinus. — This operation is not a painful 

 one. It is usually unnecessary to cast the animal, and the only 

 method of restraint required is the application of a twitch. 



From the anatomical description given, and by a further reference 



