oducing the varied
movements of the system, may be seen in the bending of the elbow.
The tendon of one extremity of the muscle is attached to the
shoulder-bone, which acts as a fixed point; the tendon of the
other extremity is attached to one of the bones of the fore-arm. When
the swell of the muscle contracts, or shortens, its two extremities
approach nearer each other, and by the approximation of the
terminal extremities of the muscle, the joint at the elbow bends.
On this principle, all the joints of the system are moved. This is
illustrated by fig. 45.

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161-172. _Give the physiology of the muscles._ 161. What are some of
the influences exerted by the muscles on the system? 162. What is
peculiar to muscular fibres? How is this illustrated? 163. Explain how
the movements of the system are effected by the contraction of the
muscles.

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[Illustration: Fig. 45. A representation of the manner in which all of
the joints of the body are moved. 1, The bone of the arm above the elbow.
2, One of the bones below the elbow. 3, The muscle that bends the elbow.
This muscle is united, by a tendon, to the bone below the elbow, (4,) at
the other extremity, to the bone above the elbow, (5,) 6, The muscle that
extends the elbow. 7, Its attachment to the point of the elbow. 8, A
weight in the hand to be raised. The central part of the muscle 3
contracts, and its two ends are brought nearer together. The bones below
the elbow are brought to the lines shown by 9, 10, 11. The weight is
raised in the direction of the curved line. When the muscle 6 contracts,
the muscle 3 relaxes and the fore-arm is extended.]

_Experiments._ 1st. Clasp the arm midway between the shoulder and
elbow, with the thumb and fingers of the opposite hand. When the arm
is bent, the inside muscle will become hard and prominent, and its
tendon at the elbow rigid, while the muscle on the opposite side will
become flaccid. Extend the arm at the elbow, and the outside muscle
will swell and become firm, while the inside muscle and its tendon at
the elbow will be relaxed.

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Explain fig. 45. Give experiment 1st.

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2d. Clasp the fore-arm about three inches below the elbow, then open
and shut the fingers rapidly, and the swelling and relaxation of the
muscles on the opposite sides of the arms, alternating with each
other, will be felt, corresponding with the