into the quick

Harper Leeの 'To Kill a Mockingbird' を読んでいます。

The boy stood up. He was the filthiest human I had ever seen. His neck was dark grey, the backs of his hands were rusty, ahd his fingernails were black deep into the quick.

"the quick" は文章からして、爪が指の皮膚と離れる個所ですね。 以前（6/26/2014）に "wound to the quick" を取り上げたことを思い出しました。　この "the quick" に会うのはそれ以来です。