DISEASES OF THE LATEEAL CARTILAGES 373 



that portion of horn forming the outer boundary of the 

 cutigeral groove, and accommodating the coronary cushion. 

 For the making of the incisions there is the special saw 

 devised for this operation by Colonel F. Smith, A.V.D., and 

 which we illustrate in Fig. 144. With this the wall is sawn 

 through imtil the depth arrived at is equal to what is indi- 

 cated by a previous examination of the thickness of the crust 

 as viewed from the solar surface. Here Colonel Smith says : 

 ' I strongly advise everyone to use a metal gauge (a thin 

 piece of material) to introduce into the incision made by the 

 saw, and run it up and down to ascertain whether the wall 

 is properly divided throughout. The depth to which this 

 should be done we know from the previous measurements 

 of our gauge on the crust.' 



Fig. 144. — Smith's Side- bone Saw (Early Pattern). 



Should the saw be of a pattern in which the set of its 

 teeth makes only a narrow incision,* it should, while opera- 

 ting, be kept well oiled, and should be withdrawn every few 

 seconds in order that the horn-dust lying in its teeth may 

 be examined. If this is getting slightly bloodstained, we 

 know, of course, that the sensitive structures are reached, 

 and the incision has been carried far enough. In so judging 

 the depth of the incision, however, care must be taken to 

 see that the top of the coronary cushion is not injured with 

 the saw, for if this is done the blood trickling into the depth 

 of the incision will tinge the horn-dust, and give the false 

 impression that the incision is sufficiently deep. 



If the operator has had no previous experience of the 

 use of the saw in this operation, he must also be careful to 



* That is Smith's older pattern. The newer pattern (Fig. 145) has 

 the teeth so set as to make an incision wide enougli to be looked into. 

 In this case the depth arrived at is to be judged by the appearance of 

 the bottom of the incision. 



