This scene depicts the propet studying intently and the composition is completed with another ribbon that curves around the exterior, just behind his head. The artist added latin lettering to this, just as he had done in some of his other supporting panel pieces. Perhaps his studio also contributed to elements of this scene, as the central panels would have been more important to him and therefore he would have wanted to manage them more closely.
In this piece the artist also puts a small amount of lettering on the inner frame too, which is somewhat unusual. Typically artists from the Renaissance would work lettering into the painting itself, perhaps aside content in a plain area of the composition, but not normally upon the frame. Van Eyck famously saw the frame as an extension to the painting itself, rather than a separate element which merely presents the artwork.
Van Eyck carefully places the figure in front of the curled ribbon before adding some shadow between the two which helps to make the man stand out as if three-dimensional. There is also another skilled use of drapery, a particular style mastered by this painter, where his portraits would appear naturally dressed with clothing realistic and hanging from the figure in a believable manner.