=The Ear.=--"The auditory organ in man consists of three portions, known respectively as the _external ear_, the _middle ear_ or _tympanum_, and the _internal ear_ or _labyrinth_; the latter contains the end-organs of the auditory nerve. The external ear consists of the expansion seen on the exterior of the head, called the _concha_, _M_, Fig. 17, and a passage leading in from it, the _external auditory meatus_, _G_. This passage is closed at its inner end by the _tympanic_ or _drum membrane_, _T_. It is lined by skin, through which numerous small glands, secreting the _wax_ of the ear, open. [Illustration: FIG. 18.--_Mcp_, _Mc_, _Ml_, and _Mm_ stand for different parts of the malleus; _Jc_, _Jb_, _Jl_, _Jpl_, for different parts of the incus. _S_ is the stapes. ] "_The Tympanum_ (_P_, Fig. 17) is an irregular cavity in the temporal bone, closed externally by the drum membrane. From its inner side the _Eustachian tube_ (_R_) proceeds and opens into the pharynx. The inner wall of the tympanum is bony except for two small apertures, the _oval_ and _round foramens_, _o_ and _r_, which lead into the labyrinth. During life the round aperture is closed by the lining mucous membrane, and the oval by the stirrup-bones. The _tympanic membrane_ _T_, stretched across the outer side of the tympanum, forms a shallow funnel with its concavity outwards. It is pressed by the external air on its exterior, and by air entering the tympanic cavity through the Eustachian tube on its inner side. If the tympanum were closed these pressures would not be always equal when barometric pressure varied, and the membrane would be bulged in or out according as the external or internal pressure on it were the greater. On the other hand, were the Eustachian tube always open the sounds of our own voices would be loud and disconcerting, so it is usually closed; but every time we swallow it is opened, and thus the air-pressure in the cavity is kept equal to that in the external auditory meatus. On making a balloon ascent or going rapidly down a deep mine, the sudden and great change of aërial pressure outside frequently causes painful tension of the drum-membrane, which may be greatly alleviated by frequent swallowing. _The Auditory Ossicles._--Three small bones lie in the tympanum forming a chain from the drum-membrane to the oval foramen. The external bone is the _malleus_ or _hammer_; the middle one, the _incus_ or _anvil_; and the internal one, the _stapes_ or _stirrup_. They are represented in Fig. 18.[19]