The _External ear_ is connected with the interior part by a prolongation of its orifice, termed the _external auditory meatus_. In man, this gristly portion of the auditory apparatus is about one inch in length, lined by a continuation of the integument of the ear, and has numerous hairs on its surface, to prevent the intrusion of foreign substances. Between the external MEATUS and the cavity of the middle ear is the _membrana tympani_, which is stretched across the opening like the head of a drum. The _tympanum_, or ear-drum, communicates with the pharynx by the _eustachian tube_, which is a narrow passage lined with delicate, ciliated epithelium. On the posterior portion it is connected with the _mastoid cells_. Three small bones are stretched across the cavity of the tympanum, and called, from their form, the _malleus, incus_ and _stapes_, or the hammer, anvil, and stirrup. Agassiz mentions a fourth, which he terms the _os orbiculare_. Each wave of sound falling upon the membrana tympani, throws its molecules into vibrations which are communicated to the chain of bones, which, in turn, transmits them to the membrane of the _foramen ovale_. The three muscles which regulate the tension of these membranes are termed the _tensor tympani, laxator tympani_, and _stapedium tympani._ The _Labyrinth_, or _Internal_ ear, is a complicated cavity, consisting of three portions termed the _vestibule, cochlea_, and _semi-circular canals_. The vestibule is the central portion and communicates with the other divisions. The labyrinth is filled with a transparent fluid, termed _perilymph_, in which are suspended, in the vestibules and canals, small membranous sacs, containing a fluid substance, termed _endolymph_ (sometimes called _vitrine auditive_ from its resemblance to the vitreous humor of the eye). The filaments of the auditory nerve penetrate the membranous tissues of these sacs, and also of those suspended at the commencement of the semi-circular canals. These little sacs are supposed to be the seat of hearing, and to determine, in some mysterious way, the quality, intensity and pitch of sounds. The determination of the _direction_ of sound is a problem of acoustics. Some have contended that the arrangement of the semi-circular canals is in some way connected with this sensation. But this supposition, together with the theory of the transmission of sound through the various portions of the cranial bones, has been exploded.