orously extend the fingers by a violent and rigid
contraction of the muscles upon the lower part of the arm, and the
lateral movement which is seen in their separation cannot be made. But
gently extend the fingers, and their oblique movements are made with
freedom.

211. An individual who is acquainted with the laws of health, whose
muscles are well trained, will perform a certain amount of labor with
less fatigue and waste to the system, than one who is ignorant of the
principles of hygiene, and whose muscles are imperfectly trained.
Hence the laboring poor have a deep interest in acquiring a knowledge
of practical physiology, as well as skill in their trade or vocation.
It is emphatically true to those who earn their bread by the "sweat of
their brow," that "knowledge is power."

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210. What is said of the lateral and oblique movements of the arm,
hand, and fingers in writing? How is this shown by experiment? 211.
Why is the study of physiology and hygiene of utility to the laborer?

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CHAPTER XII.

THE TEETH.

212. The teeth, in composition, nutrition, and growth, are different
from other bones of the body. They vary in number at different periods
of life, and, unlike other bones, they are exposed to the immediate
action of atmospheric air and foreign substances. The bones of the
system, generally, when fractured, unite; but there is never a
permanent union of a tooth when broken.

ANATOMY OF THE TEETH.

213. The TEETH are attached to the upper and lower jaw-bone, by means
of bony sockets, called _al've-o-lar_ processes. These give great
solidity to the attachment of the teeth, and frequently render their
extraction difficult. The gums, by their fibrous, fleshy structure,
serve to fix the teeth more firmly in the jaw.

_Observation._ When a _permanent_ tooth is extracted, these bony
processes are gradually absorbed, so that in advanced age there
remains only the jaw-bone covered by the lining membrane of the gum.
This accounts for the narrow jaw and falling in of the lips in old
age. Frequently, a piece of the alveolar process comes out with the
tooth when extracted, and the dentist has then the credit of "breaking
the jaw." No great injury results from the removal of the process in
this manner.

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212. What is said of the teeth? In what respect do they differ from
other bones of the body? 213-218. _Give the anato