he tendons of insertion of
these two muscles. 8, The extensor communis digitorum muscle. 9, The
extensor minimi dlgiti muscle. 10, The extensor carpi ulnaris muscle. 13,
The extensor ossis metacarpi and extensor primi internodii muscles, lying
together. 14, The extensor secundi internodii muscle; its tendon is seen
crossing the two tendons of the extensor carpi radialis longior and
brevior muscles. 15, The posterior annular ligament. The tendons of the
common extensor muscle of the fingers are seen on the back of the hand,
and their mode of distribution on the back of the fingers.

_Practical Explanation._ The muscles 5, 6, 10, extend the wrist on the
fore-arm. The muscle 8 extends the fingers. The muscle 9 extends the
little finger. The muscles 13 extend the metacarpal bone of the thumb,
and its first phalanx. The muscle 14 extends the last bone of the thumb.
The muscles 10, 13, 14, move the hand laterally.]

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160. Where are the muscles situated that effect the larger movements
of the hand? That perform the delicate movements of the fingers? Give
the use of some of the muscles represented by fig. 43. Those
represented by fig. 44.

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CHAPTER IX.

PHYSIOLOGY OF THE MUSCLES.

161. The muscles exercise great influence upon the system. It is by
their contraction that we are enabled to pursue different employments.
By their action the farmer cultivates his fields, the mechanic wields
his tools, the sportsman pursues his game, the orator gives utterance
to his thoughts, the lady sweeps the keys of the piano, and the young
are whirled in the mazy dance. As the muscles bear so intimate a
relation to the pleasures and employments of man, a knowledge of the
laws by which their action is governed, and the conditions upon which
their health depends, should be possessed by all.

162. The peculiar characteristic of muscular fibres is _contractility_,
or the power of shortening their substance on the application of
stimuli, and again relaxing when the stimulus is withdrawn. This is
illustrated in the most common movements of life. Call into action the
muscles that elevate the arm, by the influence of the _will_, or mind,
(the common stimulus of the muscles,) and the hand and arm are
raised; withdraw this influence by a simple effort of the will, and
the muscles, before rigid and tense, become relaxed and yielding.

163. The contractile effect of the muscles, in pr