movement of the fingers.
While the fingers are bending, the inside muscles swell, and the
outside ones become flaccid; and, while the fingers are extending, the
inside muscles relax, and the outside ones swell. The alternate
swelling and relaxation of antagonist muscles may be felt in the
different movements of the limbs.

164. Each fibre of the several muscles receives from the brain, through
the nervous filament appropriated to it, a certain influence, called
nervous fluid, or stimulus. It is this that induces contraction, while
the suspension of this stimulus causes relaxation of the fibres. By
this arrangement, the action of the muscular system, both as regards
duration and power, is, to a limited extent, under the control of the
mind. The more perfect the control, the better the education of the
muscular system; as is seen in the graceful, effective, and
well-educated movements of musicians, dancers, skaters, &c.

165. The length of time which a muscle may remain contracted, varies.
The duration of the contraction of the voluntary muscles, in some
measure, is in an inverse ratio to its force. If a muscle has
contracted with violence, as when great effort is made to raise a
heavy weight, relaxation will follow sooner than when the contraction
has been less powerful, as in raising light bodies.

166. The velocity of the muscular contraction depends on the will.
Many of the voluntary muscles in man contract with great rapidity, so
that he is enabled to utter distinctly fifteen hundred letters in a
minute; the pronunciation of each letter requiring both relaxation and
contraction of the same muscle, thus making three thousand actions in
one minute. But the contraction of the muscles of some of the inferior
animals surpasses in rapidity those of man. The race-horse, it is
said, has run a mile in a minute; and many birds of prey will probably
pass not less than a thousand miles daily.

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Give experiment 2d. 164. With what is each muscular fibre supplied?
What effect has this stimulus on the muscles? 165. how long does a
voluntary muscle remain contracted? 166. On what is the velocity of
muscular contraction dependent? How many letters may be pronounced in
a minute?

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167. The functions of the involuntary muscles are necessary the
digestion of food, the absorption and circulation of the nutritive
fluids. They could not be trusted with safety to the control of