Sentence:  The polyhedron "P"∪"D" is a special <WSD>spine</WSD> of the twice punctured "M", that is, of "M" with two balls "B"<sub>1</sub>, "B"<sub>2</sub> cut out of it. To get a <WSD>spine</WSD> of "M", we make a hole in "c"′ or "c"″ depending on which of them is a common face of these balls. 
Meaning of the word marked with <WSD> tags:  Something resembling a backbone, such as a ridge, or a long, central structure from which other structures radiate.