The structure composed of a sheet of simple squamous epithelial cells that folded onto itself to form a double layer is

a. nuclear membrane
b. parietal layer
c. mucous membrane
d. serous membrane

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.