cluster of navigation bars in the MASTERBORDER or at the end of each page provides access to parent textual chapters at tier 2, and to visual aids and pictorials at tier 3. There are four pictorials listed under TEXT 4-X (SxtrC12); the four pictorials are children of TEXT 4-X. In navigations along tier 3, movements back or forward using the BACK or NEXT button
would be limited by the relationships between parent and children. If the reader were at tier 3 and viewing a photomicrograph on Chapter 14, only Pictorial Chapters 13, 14, 15, and 17 would be accessible, if
navigation were to be limited to movement provided by the BACK, or the NEXT buttons at the end of each page.