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CUTTING INSTRUMENTS. 31

generally by careless handling. A certain degree of stiffness is
essential in the shanks of excavators. They should be no larger
than is actually recjuired for this. Within the range of exact
observation, thirty-five pounds lateral pressure on an instru-
ment point is the limit that any man has been able to make with
the pen grasp. For a large proportion of the smaller blades,
twenty poimds is abundant. Indeed, it is only a small minority
of men who can exert that force with the pen grasp. The only
possible need of stronger instruments nowadays is in the pos-
sible use of the palm-and-thumb grasp, which is limited to a
very few instrument forms. This allows us to use instruments
of very delicate forms, that are also elegant, and obscure the
following of the work by the eye to the least possible amount.
Rights and lefts. "Wliile the greater number of cutting
instruments are direct cutting, there is a distinct division of
lateral cutting instruments better known as "rights and lefts."
These are all double plane instruments. If an instrument with
two or more curves or angles in the shank is laid on a table, it
will generally be found that all of the curves are in a single
plane, which may be made to coincide with the plane of the
table top. Or, if held before the eye, or between the eye and a
window, in a certain position in relation to its curves, it forms
a straight line. This is a single plane instrument because all
of its angles and curves are on one plane. All of the lateral
cutting instruments have an angle or curve in a plane at right
angles to this principal plane. This will be seen by holding any
of the spoons or gingival margin trimmers before the eye. The
curve of the blade is in the opposite plane to the angles. It is
this that makes it a lateral cutting instrument. They are there-
fore double plane instruments. Angles that are in any other
direction than in one of these planes are inadmissible. It often
appears that an angle in a different direction would enable one
to do certain things easier and many instruments have been so
made. None of them have remained long in use because of their
unfitness. There seems to be some undefined sense of antag-
onism to the muscular sense or the appreciation of motion that
causes all such instrument forms to become awkward and
obnoxious.
BiBEVELED CUTTING INSTRUMENTS. lu the Ordinaries, the
hatchets are all bibeveled cutting instruments intended for direct
cutting. In forming the edges, the blades are equally beveled
on the two sides. In sharpening these, they should be ground
equally on the two sides. They cut by being pushed forward