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Internally, a muscle fiber has lengthwise
myofibrils, each composed of a series of segments called sarcomeres.
Each sarcomere contains many lengthwise myofilaments. The two types
of myofilaments (thick & thin) occur in a regular arrangement which
causes the cross striations (alternating dark and light bands) to be
seen in myofibrils and fibers. These bands are readily seen in the
image to the left.
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