Most species also have a thin, partly clear inner eyelid attached to the bottom lid. This inner eyelid, called the nictitating membrane, can be moved upward when a frog's eyes are open. It protects the eyes without completely cutting off vision. Most frogs have a disk of skin behind each eye. Each disk is called a tympanum, or eardrum.
Sound waves cause the eardrums to vibrate.
The vibrations travel to the inner ear, which is connected by nerves to the hearing centers of the brain. Most frogs have a delicate sense of touch. It is particularly well developed in species that live in water. The tongue and mouth have many taste buds, and frogs often spit out bad-tasting food. The sense of smell varies among species.