520 DISEASES AND INJUKIES OF THE FACIAL REGION. 



mouth, as the teeth are not always relatively in the same posi- 

 tion as regards the termination of the zygomatic ridge. 



To trephine successfully, it is necessary to cast the animal. 

 I have operated without casting, but the operation is better 

 performed when the horse is down. 



It is advisable to remove a round piece of the skin and sub- 

 cutaneous tissue, rather larger than the size of the trephine, and 

 to dissect down to the bone cleanly, removing all shreds of the 

 soft structures, or the trepliine will become clogged with debris ; 

 and, to render the operation less difficult, it is a good plan to 

 bore a small hole in the centre of the piece of bone with a small 

 simlet, and to introduce the centre-bit of the trephine into the 

 hole so bored. 



The best kind of trephine is that with a moveable centre-bit ; 

 but the latest improved trephines made by Mr. Mackenzie being 

 very superior instruments, need no description. 



AVhen the piece of bone is sawn through, it will, in the 

 majority of instances, come out of the aperture along with the 

 saw, or it may fall into the sinus ; and when it is attached to 

 the dividing ridge of the two sinuses, it may be necessary to 

 give the trephine a slight twist to one side, to disconnect the 

 trephined portion from the bony plate. 



The additional instruments are a strong scalpel, a pair of 

 forceps, and an elevator, to raise the piece of bone when this is 

 not accomplished with the trephine. This elevator, made of 

 steel, should be six or seven inches long, from half to three 

 quarters of an inch wide at one extremity, which is to be cir- 

 cular, to correspond to the trephine, and sufficiently thin to pass 

 into the groove made by the saw. 



Mr. Yarnell recommends that the skin should be divided by 

 a crucial incision, and the sinuses explored by the probe. As 

 to the crucial incision, I find that the flaps so made are a 

 disadvantage, interfering with the necessary daily washing and 

 dressing of the sinuses ; that they become inflamed, painful, and 

 swollen, and cause the animal to be restive when an attempt is 

 made to dress the part ; and the wound is also apt to close too 

 quickly if its margins are not destroyed by caustic — a bad 

 practice. In making the circular incision, and removing the 

 whole of the tissues contained within the circle, the operation is 

 more easily performed, the edges of the wound do not interfere 



