upper jawbone of the beak; not fused to the skull as in humans, but articulates (forms a joint) with it dorsally behind the cere (in Bald Eagles), forming a hinge so that the maxilla can move upward; extends under the skull with the palatine (forming the palate) and the jugal arch, which articulates below the ear with a bone (the quadrate) that also articulates with the lower beak (mandible), opening and closing both lower and upper bones simultaneously; bone is covered by a layer of tissue with blood vessels and nerves, and sheathed on top by a thick hard covering of beta-keratin, which is harder than the alpha-keratin found in mammals; sometimes called the upper mandible; photo of skull © Idaho Virtual Museum (lower mandible not shown), my labels added3D VISUALIZATION OF A BALD EAGLE SKULL