ORGAN OF HEARING.
The organ is divided into two parts, the external and internal ear, by the membra.
sa tympani. The situation of the former on the out side of the heial is well known; the latter is contained in the petrous por tion of the temporal hone.
The external ear consists of two parts, /tit-- the pinna, or ear, popularly so called, and a tube called meatus auditorius ex tennis leading from the pinna to the mem. brans, tympani. These parts serve for col lecting sou nds, and conveyingthem to the membrana ty-mpani.
The pinna consists of a convoluted car tilage inclosed by common integuments. The lower part, which is pierced for ear rings, 11a5 no cartilage, and is called the lobulus. The helix is the fold formingthe external circumference of the ear ; the next eminence to this, which forms the margin of the great cavity of the external ear, is called anthelix ; it separates at its u pper and anterior end into two processes named crura. The projection immediate ly in front ofthe meatus isthe traps, and that immediately opposite, the antitragus. The great cavity within the anthelix, and leading to the meatus, is called the con cha. Several sebaceous glands are situa ted in the folds of the ear.
The meatus externus is formed first by a portion of cartilage, continued from the pinna, and more interiorly it consists of a canal in the substance of the bone. This bony part does not exist in the fcetus, where the meatus is wholly clutilaginons. The common integumentscontinued from the pinna line the meatus externu.s, and the ci icicle is produced over the membra na tympani.
The surface of the meatus, at its com mencement, is fbrnislied with numerous fine hairs, and the canal is moistened by-a secretion of an oily and inflammable na ture, called cerumen. This is produced by numerous small glands, visible on the external surface of the meatus, and dis tinguishable hy their yellowish colour. The cerumen concretes, and is collected sometimes in such quantity as to induce a slight degree ofdeafness, which is easily removed by syringing tvith warm water.
The membrana tympani, which is a cir cular membrane 1110VC a quarter ofan inch in diameter, is stretched across the inner extremity of the meatus, and derives its 11301t!from a comparison with A drum head, to which it hears scnneanalog-y in its use. in the fIxttis it is stretched on a distinct ban.- ring, called the annulus auditorius.
Thii ring is deficient at its up perpart, a.nd has no bony union to the rest of the tem poral bone, but it becomes united soon after birth. _ _ This membrane is concave on its exte rior surffice, and convex towards the tin). panum. I ts position is inclined, the upper margin being more towards the outside of the head, and the under part farther in wards ; so that the superior part of the meatus forms an obtuse angle, and the in ferior part an acute angle, with the mem brane.
The internal ear consists of two diNi sions, viz. the tympanutu and the laby rinth.
The tympanum is an irregular bony ca vity, which will about admit the end of a finger, hollowed out ofthe temporal bone, just within the membrana tympani. It has several communications with the neigh bouring parts.
Opposite to the membrana tympani are two openings, which lead to the labyTinth ofthe ear. The upper one is named the fenestra ovalis, the lower one the fenestra rotunda, and the projection between them is called the promontory. The fenestra ovalis is filled, as we shall presently see, by one of the little bones of the tympa num, and the fenestra rotunda is closed by *membrane.
The eustachian tube, or iter a pahtto ad attrem, opens in front of the tympanum. It commences by an expanded cartilagi nous orifice at the back of the nostrils, passes through tite substance of the tem poral bone, and terminates by a contract ed orifice in the tympanum. Its office is to convey air into the cavity of the tympa num. The membrana tympani is thrown into vibrations by the impulse of the sono rous undulations of the air, and that vibra tion could not take place unless there was air in the inside as well as on the outside of the membrane. Water, or any other fluid, would not have answered the pur pose. Hence an obstruction of this tube causes deafness, which surgeons ha% e tempted to remedy by punctitring the membrana tympani. An opening In the latter membrane of a small extent does by no means injure hearing ; for many per sons have the powerof Impelling tobacco smoke, or agitating the flame of a candle, through the ear, and yet seem to have a perfect use of the organ. In these cases the air or smoke enters the eustachian tube from the throat, and passes through the unnatural aperture in the membrane.