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 until the depth arrived at is equal to what is indicated 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.’

[Illustration:  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the set of its teeth makes only a narrow incision,[A] it should, while operating, be kept well oiled, and should be withdrawn every few seconds in order that the horn-dust lying in its teeth may be examined.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 blood-stained, 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.

[Footnote A:  That is Smith’s older pattern.  The newer pattern (Fig. 145) has the teeth so set as to make an incision wide enough to be looked into.  In this case the depth arrived at is to be judged by the appearance of the bottom of the incision.]

If the operator has had no previous experience of the use of the saw in this operation, he must also be careful to avoid placing too great a pressure on the teeth of its lower third.  This is done by keeping the hand too greatly depressed.  Again, this leads to wounding of the sensitive structures (this time at the lower end of the incision), and again the operator is confused by the blood thus allowed to run into the groove.

The only portion of horn difficult to operate on is that immediately under the coronet.  This is best severed with a succession of downward movements, and is easier performed with Smith’s later pattern of side-bone saw (Fig. 145) in which the set of the foremost teeth is reversed.

[Illustration:  FIG. 145.—­SMITH’S SIDE-BONE SAW (IMPROVED PATTERN).]

In making these grooves we must say that we think the use of the special saw may be dispensed with, and the incisions just as easily, or, at any rate, just as successfully, made with the knife.  Those who select to use this instrument should choose a narrow-topped and sharp searcher, or a modern shaped drawing-knife of suitable size, such as those depicted in Fig. 46, a and b, and they will find their work much easier if they will make the first steps in the incisions with an ordinary flat firing-iron.  By the use of the latter instrument the grooves are made conveniently open along their tops, and room left for nicely finishing the more delicate manner of removing with the knife the softer horn near the sensitive structures.