The cavity of the chest in which the lungs are suspended is also lined by the pleura.

A limpid fluid exudes from the pleura which keeps it moist, so that when the two surfaces rub together, as the lungs move, they do not become chafed and irritated.
~19.

Walls of the Chest.~--The ribs form a part of the framework of the chest.

The ribs are elastic.

The spaces between them are filled up with muscles, some of which draw the ribs together, while others draw them apart.