Adouble-walled sac called the encases the heart, which serves to protect the heart and anchor it inside the chest. vena cava pericardium aorta septum

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.