“Let me speak to him… face-to-face…”
“Master, you are not strong enough!”
“I have strength enough… for this…”
Harry felt as if Devil’s Snare was rooting him to the spot. He couldn’t move a muscle. Petrified, he watched as Quirrell reached up and began to unwrap his turban. What was going on? The turban fell away. Quirrell’s head looked strangely small without it. Then he turned slowly on the spot.
Harry would have screamed, but he couldn’t make a sound. Where there should have been a back to Quirrell’s head, there was a face, the most terrible face Harry had ever seen. It was chalk white with glaring red eyes and slits for nostrils, like a snake.
“Harry Potter…” it whispered.
(Harry Potter and the Sorcerer's Stone)
It seems, I think, ‘the’ should be put instead of ‘a’, for the owner of the back is specified. Then why is there ‘a’?