Birds possess distinct characteristics that make them one of the most distinguishable group of vertebrates. Feathers provide insulation and enable flight (feathers are modifications of a bird's epidermis). Birds do not possess teeth or the heavy jawbones seen in other vertebrates. Instead, birds have a pair of toothless mandibles that are covered with a horny sheath of keratin (this sheath that covers the mandible is also called the ramphotheca). Birds have a furcula, a bone that is also known as the 'wishbone'. The furcula is a bone located in the bird's chest that prevents compression of the chest cavity during the downstroke of a wingbeat.