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PROXIMAL CAVITIES IN BICUSPIDS AND MOLARS 61
to join the gingival portion of the outline by the use of a seg-
ment of a small circle. The use of a large circle here is a most
common error. Investigation of fillings will shoAV many failures
Avherein a large circle has been used allowing the external outline
to disappear in the proximal space before it has disappeared be-
neath the gum.
The Gingival Outline should be a straight outline except in well
defined and high gum festoons, Avhen it may be made convex to the
occlusal surface.
Forming the Step. Place a small round bur or spade drill against
the axial Avail at the dento-enamel junction, immediately beloAV
the central fissure and undermine the enamel the desired distance
in the direction of the central axial line of the tooth. Here apply
all of the rules and methods of procedure given in the formation

^^^^^^^^^^^^^^^^^^^^^^^^^B^' ^^^^^^^^^^^^^^^^^^^^

A B
Fig. 24.—Class Two cavities in molar and bicuspid suitable for cohesive gold or amalgam.

of a simple occlusal cavity. Also remember to apply the rules as
given in outline form, particularly as to resistance form.
Area Included. In addition to the above it is a safe rule to state
that the step portion should involve the central third of the oc-
clusal surface bucco-lingually.
Avoid all Angles in outline. Care should be taken -\Ahen us-
ing the step that its union Avith the cavity proper does not shoAv
in the outline by an angle at their junction. Also Avhen not us-
ing the step, as in the few eases cited, care should be given not to
alloAV the axio-buccal and axio-lingual line angles to meet the ex-
ternal enamel line. These line angles should be stopped before
they approach the enamel wall.