A pair of folds is found at the bottom of the vestibule of the larynx. These are called the vocal folds or true vocal cords. Extending from front to back, there is one vocal fold on each side. With a special set of muscles, the vocal folds can be drawn apart or pulled together, altogether forming the glottis.
Thus, the vocal folds are used to control the size of the opening between them, which is called the rima glottidis. When the rima glottidis is wide, air can flow easily between the upper and lower air passageways. When the vocal cords are drawn so tightly that the rima glottidis is completely closed, no air can flow through.
In Valsalva's maneuver, the lungs are filled with air and the rima glottidis is closed tightly. The muscles of the trunk wall contract strongly to increase the internal pressure of the trunk.
This internal pressure stiffens the trunk into a more rigid structure. Thus, one uses Valsalva's maneuver to provide support for a strenuous effort with the upper members. When Valsalva's maneuver is followed by a sudden opening of the rima glottidis, the result is a cough. This is used to clear the air passageways. An individual whose trunk wall muscles are paralyzed cannot do these things.