You are looking at medieval parchment - animal skin - that was stabbed, cut and stitched up. Preparing animal skin, the first step in producing a medieval book, was challenging. The parchment maker had to scrape off the fleshy bits from the one side, and the hair from the other... If the parchment maker pushed too hard while removing the unwanted parts, he would cut right through the surface, which is what happened in the images above. While some cuts were simply stitched up with a thin parchment cord, it also happened that book producers turned these defects into art. I tumbled these images from a manuscript in Uppsala a while ago, in which holes are plugged with embroidery... In the last image the material was used to attach a missing corner, producing what I tend to call a “Frankenstein page”.From Erik Kwakkel's incomparable blog about medieval books; there are two additional images of repaired pages at the link.
See also this post.