=Accommodation= is provided for in the ear as well as in the eye. One muscle an inch long, the _tensor tympani_, arises in the petrous portion of the temporal bone (running in a canal parallel to the Eustachian tube) and is inserted into the malleus below its head. When it contracts, it makes the membrane of the tympanum more tense. Another smaller muscle, the _stapedius_, goes to the head of the stirrup-bone. These muscles are by many persons felt distinctly contracting when certain notes are heard, and some can make them contract at will. In spite of this, uncertainty still reigns as to their exact use in hearing, though it is highly probable that they give to the membranes which they influence the degree of tension best suited to take up whatever rates of vibration may fall upon them at the time. In listening, the head and ears in lower animals, and the head alone in man, are turned so as best to receive the sound. This also is a part of the reaction called 'adaptation' of the organ (see the chapter on Attention). =The Internal Ear.=--"The labyrinth consists primarily of chambers and tubes hollowed out in the temporal bone and inclosed by it on all sides, except for the oval and round foramens on its exterior, and certain apertures for blood-vessels and the auditory nerve; during life all these are closed water-tight in one way or another. Lying in the _bony labyrinth_ thus constituted are membranous parts, of the same general form but smaller, so that between the two a space is left; this is filled with a watery fluid, called the _perilymph_; and the _membranous internal ear_ is filled by a similar liquid, the _endolymph_. [Illustration: FIG. 19.--Casts of the bony labyrinth. _A_, left labyrinth seen from the outer side; _B_, right labyrinth from the inner side; _C_, left labyrinth from above; _Co_, cochlea; _V_, vestibule; _Fc_, round foramen; _Fv_, oval foramen; _h_, horizontal semicircular canal; _ha_, its ampulla; _vaa_, ampulla of anterior vertical semicircular canal; _vpa_, ampulla of posterior vertical semicircular canal; _vc_, conjoined portion of the two vertical canals. ] =The Bony Labyrinth.=--"The bony labyrinth is described in three portions, the _vestibule_, the _semicircular canals_, and the _cochlea_; casts of its interior are represented from different aspects in Fig. 19. The vestibule is the central part and has on its exterior the oval foramen (_Fv_) into which the base of the stirrup-bone fits. Behind the vestibule are three bony semicircular canals, communicating with the back of the vestibule at each end, and dilated near one end to form an _ampulla_. The bony cochlea is a tube coiled on itself somewhat like a snail's shell, and lying in front of the vestibule.