The bone and cartilage that divides the inside of the nose in half is called the nasal septum. The bone and cartilage are covered by a special skin called a mucous membrane that has many blood vessels in it. Ideally, the left and right nasal passageways are equal in size. However, it is estimated that as many as 80 percent of people have a nasal septum that is off-center. This is called a deviated septum, which may or may not cause certain symptoms.