Sentence:  Thus every part and plane of the picture keeps changing place in relative depth with every other part and plane ; and it is as if the only stable relation left among the different parts of the picture is the ambivalent and ambiguous one that each has with the <WSD>surface</WSD> . 
Meaning of the word marked with <WSD> tags:  the outer boundary of an artifact or a material layer constituting or resembling such a boundary