The thread and sewing supports holding this book together have broken, and the sections that make up the textblock are detaching. After a long life of several hundred years, especially with heavy handling, even high-quality thread and sewing supports can break.
Split Textblock on a 12th Century Manuscript
Nicolaus Gospels: manuscript, 1133. Special Collections Codex Manuscripts: alc Ms129. Photo by Melina Avery
Detached Cover and Broken Textblock in a 12th Century Manuscript
Nicolaus Gospel: manuscript, 1133. Special Collections Codex Manuscripts: alc Ms129. Photo by Melina Avery