求翻译：StructureA scroll is usually divided up into pages, which are sometimes separate sheets of papyrus or parchment glued together at the edges, or may be marked divisions of a continuous roll of writing material. The scroll is usually unrolled so that one page is exposed at a time, for writing or reading, with the remaining pages rolled up to the left and right of the visible page. It is unrolled from side to side, and the text is written in lines from the top to the bottom of the page. Depending on the language, the letters may be written left to right, right to left, or alternating in direction (boustrophedon).是什么意思？

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