The floor of the oral cavity proper is formed mainly by three structures: a muscular diaphragm, which fills the U-shaped gap between the left and right sides of the body of the mandible and is composed of the paired mylohyoid muscles; two cord-like geniohyoid muscles above the diaphragm, which run from the mandible in front to the hyoid bone behind; and the tongue, which is superior to the geniohyoid muscles.