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300 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.
Palatal. Pertaining to or directed toward the palate. "Palatal surfaces of
the teeth." Obsolete.
Palm-andthumb grasp. The instrument shaft is grasped in the palm of the
hand and the working point brought into opposition to the thumb as a counteracting
force. Used in positions in which an instrument can be applied to one side or portion
of a tooth, or other object, while the thumb of the same hand is rested upon it, or
upon adjacent teeth. See Figure 29, Vol. 2.
Palm thrust grasp. The end of the instrument handle is placed in the hollow
of the palm of the hand and grasped by the fingers. Used for making a powerful
thrust. See Figure 30, Vol. 2.
Pen grasp. A grasp of dental instruments closely resembling the usual grasp
of a pen used in writing. In the use of dental instruments the pulps of the thumb,
first and second fingers should be on the instrument shaft. See Figure 26, Vol. 2.
Pericementum. The membrane covering the cementum of the roots of teeth.
See Peridental Membrane.
Peridental membrane. The membrane surrounding the root of a tooth and
forming its attachment to the walls of its alveolus. It is composed of fibers which
are built into the cementum on the one side and into the bone on the other. Among
these fibers there is an abundant network of blood vessels, nerves and cellular ele-
ments.
Periosteal boxe. Bone laid down by the osteoblasts of the periosteum as dis-
tingushed from Haversian systems bone.
Permanent teeth. The teeth of adult age as distinguished from the temporary,
or deciduous teeth.
Permanent teeth, childhood period of. The period from the first appearance
of the permanent teeth until their roots are fullv completed, except the third molars.
See Figure 187, Vol. 1.
Personal equation. When one person accomplishes any certain result, which
another fails, or in part fails to do, and the differences in opportunity and method
seem inexplicable, the difference is said to be a personal equation. As these differ-
ences are being occasionally explained by more acute observation or by physical meas-
urements, they are found to be due to personal physical conditions, or to differences
in mental grasp of requirements.
Phagodynamometer. An instrument by which the force required in crushing
different examples of foodstuffs is determined. See Figure 166, Vol. 1.
Photomicrograph. A photograph of a tissue or other prepared object taken by
transmitted light with a microscopic lens. This should be distinguished from a
photograph made by reflected light by a microscopic lens of low power such as the
photograph of a tooth in which the object is enlarged from two to eight or ten diam-
eters. Photomicrographs may have an enlargement anywhere from two or three
diameters to a thousand or more.
Pit. A sharp, pointed depression in the enamel. Pits occur mostly where several
developmental grooves join; as in the occlusal surfaces of the molars, at the endings
of the buccal grooves on the buccal surfaces of the molars; occasionally in the lin-
gual surfaces of the incisors.
Pit cavities. Those cavities that begin in pits. Cavities formed by decay that
begins in pits of any kind or in any position. See Figures 68, 69, 70, 71, 75, 76, 77,
Vol. 1.
Planes of the teeth. Any planes cutting the teeth in definite directions, as
the axial plane, horizontal plane, etc.
Plaque. A flat circumscribed area or plate; a flat plate made up of a colony
of microorganisms in zooglea form, clinging to enamel of the teeth.
Plastics. Materials like cements, gutta-percha and amalgam for filling cavities
in teeth ; a material for any purpose that may be easily modeled and built into form.