The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the inner ear. Also mammalian middle ear contains three ossicles, which transfers the vibrations of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has also been called the tympanic cavity, or cavum tympani. The eustachian tube (also known as the auditory tube or the pharyngotympanic tube) joins the tympanic cavity with the nasal cavity (nasopharynx), allowing pressure to equalize between the middle ear and throat.