of it. By these processes, the food is prepared to
pass more easily and rapidly into the stomach.

220. In the mastication of food there are two movements of the lower
jaw--the action by which the teeth are brought together, and the
lateral motion. In the former, the food is cut or divided, the jaws
acting like shears. This movement is produced by the action of two
large muscles situated on each side of the head and face.

_Observation._ The muscles attached to the lower jaw are of great
strength; by their action alone, some persons are enabled to bite the
hardest substances. By putting the fingers upon the side of the head
above and in front of the ears, and upon the face above the angle of
the jaw, while masticating food, the alternate swelling and relaxation
of these muscles will be clearly felt.

221. The lateral, or grinding movement of the teeth, is produced by
the action of a strong muscle that is attached to the lower jaw on the
inside.

_Observation._ Those animals that live solely on flesh, have only the
cutting, or shear-like movement of the jaws. Those that use
vegetables for food, have the grinding motion; while man has both the
cutting and grinding movement.

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219-222. _Give the physiology of the teeth._ 219. Give one of the
functions of the teeth. 220. How many movements of the lower jaw in
masticating food? What effect has the first movement upon the food?
How produced? What is the character of the masticating muscles? 221.
How is the grinding motion of the teeth produced? What is said of the
movements of the teeth in different animals?

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222. 2d. The teeth aid us in articulating with distinctness certain
letters and words. An individual who has lost his front teeth cannot
enunciate distinctly certain letters called dental. Again, as the
alveolar processes are removed by absorption soon after the removal of
the teeth, the lips and cheeks do not retain their former full
position, thus marring, in no slight degree, the symmetry of the lower
part of the face. Consequently, those simple observances that tend to
the preservation of the teeth are of great practical interest to all
persons.

HYGIENE OF THE TEETH.

223. _To preserve the teeth, they must be kept clean._ After eating
food, they should be cleansed with a brush and water, or rubbed with a
piece of soft flannel, to prevent the _tartar_ collecting, and to
remove the pieces of food t