Layer of the Sac Surrounding a Lung
Puzzle 7 of Dungeon Delving Pack
Question is: Layer of the Sac Surrounding a Lung
and the answer is: PLEURA
Word Meaning:
PLEURA — (anatomy) Each of a pair of smooth serous membranes which line the thorax and envelop the lungs in humans and other mammals.
Definitions from Wiktionary.