A Treatise on Anatomy, Physiology, and Hygiene Part 61

947. The MEATUS AUDITORIUS is a ca.n.a.l partly cartilaginous, and partly bony, about an inch in length, which extends inward from the pinna to the _Membra-na Tympan-i_, (drum of the ear.) It is narrower in the middle than at the extremities. It is lined by an extremely thin pouch of cuticle, which, when withdrawn, after maceration, preserves the form of the ca.n.a.l. Some stiff, short hairs are also found in the interior of the channel, which stretch across the tube, and prevent the ingress of insects. Beneath the cuticle are a number of small follicles, which secrete the wax of the ear.

[Ill.u.s.tration: Fig. 144. A representation of the four bones of the ear.

The smallest is highly magnified. This bone is early matured, and in the adult it becomes united with the incus. These bones are retained in their places and moved by three ligaments and four muscles.]

948. The MEMBRANA TYMPANI is a thin, semi-transparent membrane, of an oval shape. It is about three eighths of an inch in diameter, and is inserted into a groove around the circ.u.mference of the meatus, near its termination. This membrane is placed obliquely across the area of that tube. It is concave toward the meatus, and convex toward the tympanum.

949. The TYMPANUM consists of an irregular bony cavity, situated within the temporal bone. It is bounded externally by the membrana tympani; internally by its inner wall; and in its circ.u.mference by the petrous portion of the temporal bone and mastoid cells. The tympanum contains four small bones, called the _os-sicu-la au-ditus_. These are named separately, the _malle-us_, _incus_, _stapes_, and _or-bicu-lar_.

947. What is the meatus auditorius? What is found in this ca.n.a.l? What is their use? Where is the wax of the ear secreted? 948. Describe the membrana tympani. 949. Where is the tympanum situated?

950. There are ten openings in the middle ear; five large and five small. The larger openings are, the _Me-atus Aud-it-ori-us Ex-ternus_, _Fe-nestra O-valis_, (oval window,) _Fe-nestra Ro-tunda_, (round window,) _Mastoid Cells_, and _Eu-stachi-an Tube_.

[Ill.u.s.tration: Fig. 145. A representation of the pinna, meatus, membrana tympani, bones of the ear, and semicircular ca.n.a.ls. _a_, The pinna. _c_, The meatus auditorius externus. _g_, The membrana tympani. _k_, The tympanum. _e_, The bones of the ear. _b_, The semicircular ca.n.a.ls. _f_, The cochlea. _h_, The vestibule. _i_, The Eustachian tube. _d_, The auditory nerve.]

951. The FENESTRA OVALIS is the opening of communication between the tympanum and the vestibule. It is closed by the foot of the stapes, or bone of the ear, and by the lining membrane of both cavities.

952. The FENESTRA ROTUNDA serves to establish a communication between the tympanum and the cochlea. it is closed by a proper membrane, as well as by the lining of both cavities.

What does this cavity contain? 950. How many openings in the tympanum?

Explain fig. 145. 951. Describe the fenestra ovalis. 952. The fenestra rotunda.

953. The MASTOID CELLS are very numerous, and occupy the whole of the interior of the mastoid process of the temporal bone, and part of the petrous bone. They communicate, by a large, irregular opening, with the upper and posterior circ.u.mference of the tympanum.

[Ill.u.s.tration: Fig. 146. A view of the labyrinth laid open. This figure is highly magnified. 1, 1, The cochlea. 2, 3, Two channels, that wind two and a half turns around a central point, (5.) 7, The central portion of the labyrinth, (vestibule.) 8, The foramen rotundum. 9, The fenestra ovalis. 11, 12, 13, 14, 15, 16, 17, 18, The semicircular ca.n.a.ls. The cochlea and semicircular ca.n.a.ls open into the vestibule.]

954. The EUSTACHIAN TUBE is a ca.n.a.l of communication, extending obliquely between the pharynx and the anterior circ.u.mference of the tympanum. In structure it is partly fibro-cartilaginous and partly bony. It is broad and expanded at its pharyngeal extremity, and narrow and compressed at the tympanum.

953. Where are the mastoid cells? Explain fig. 146. 954. Describe the Eustachian tube.

955. The small openings of the middle ear are for the entrance and exit of the chorda tympani, (a small nerve that crosses the tympanum,) and for the exit of the muscles that act upon the membrana tympani and bones of the ear.

956. The LABYRINTH consists of a membranous and a bony portion. The bony labyrinth presents a series of cavities which are channelled through the substance of the petrous bone. It is situated between the cavity of the tympanum and the _Audit-o-ry Nerve_. The labyrinth is divided into the _Vesti-bule_, _Sem-i-circu-lar Ca.n.a.ls_, and _Cochle-a_.

957. The VESTIBULE is a small, three-cornered cavity, situated immediately within the inner wall of the tympanum.

958. The SEMICIRCULAR Ca.n.a.lS are three bony pa.s.sages which communicate with the vestibule, into which two of them open at both extremities, and the third at one extremity.

959. The COCHLEA forms the anterior portion of the labyrinth. It consists of a bony and gradually tapering ca.n.a.l, about one and a half inches in length, which makes two turns and a half, spirally, around a central axis, called the _mo-dio-lus_. The modiolus is large near its base, where it corresponds with the first turn of the cochlea, and diminishes in diameter toward its extremity.

960. The interior of the ca.n.a.l of the cochlea is partially divided into two pa.s.sages, by means of a bony and membranous plate. At the extremity of the modiolus, the two pa.s.sages communicate with each other. At the other extremity, one opens into the vestibule; the other into the tympanum, by the foramen rotundum. The internal surface of the bony labyrinth is lined by a fibro-serous membrane.

955. What pa.s.ses through the small openings of the middle ear? 956. Of what does the labyrinth consist? Give the parts of the internal ear.

957. Describe the vestibule. 958. What is said of the semicircular ca.n.a.ls? 959. Why is the cochlea so called? Of what does it consist?

960. How is the interior of the ca.n.a.l of the cochlea divided? Where do they communicate with each other?

961. The membranous labyrinth is smaller in size, but a perfect counterpart, with respect to form, of the bony vestibule, cochlea, and semicircular ca.n.a.ls. Within this labyrinth are two small, elongated sacs, which are filled with a fluid.

[Ill.u.s.tration: Fig. 147. A view of the auditory nerve. 1, The spinal cord. 2, The medulla oblongata. 3, The lower part of the brain. 4, The auditory nerve. 5, A branch to the semicircular ca.n.a.ls. 6, A branch to the cochlea.]

962. The AUDITORY NERVE enters the temporal bone upon its internal surface, and divides into two branches, at the bottom of the cavity of the internal ear. These branches enter the structure of the elongated sacs and membranous labyrinth, radiating in all directions, and finally, they terminate upon the inner surface of the membrane, in minute papill, resembling those of the retina.

By what is the internal labyrinth lined? 961. Describe the membranous labyrinth. What does fig. 147 represent? 962. Where does the auditory nerve enter and divide? Where do the branches of the auditory nerve enter and terminate?

CHAPTER XLVII.

PHYSIOLOGY OF THE ORGANS OF HEARING.

963. HEARING is that function by which we obtain a knowledge of the vibratory motions of bodies, which const.i.tute sounds. The precise function of all the different parts of the ear is not known.

964. The function of that part of the external ear which projects from the head is to collect sounds and reflect them into the meatus.

965. The membrana tympani serves to facilitate the transmission of sounds, and also to moderate their intensity. It is so arranged that it can be relaxed or tightened.

_Observation._ This membrane, when healthy, has no opening; and it must be apparent that the apprehension which is often expressed, that insects will penetrate further, is groundless. The pain is owing to the extreme sensibility of the membrana tympani.

966. The supposed office of the tympanum is to transmit the vibrations made on the membrana tympani to the internal ear. This is effected by the air which it contains, and by the chain of small bones that are enclosed in this cavity.

967. The use of the Eustachian tube is to admit air into the tympanum, which renders the pressure on both sides equal, and thus its membrane is kept in a proper state of tension.

963-971. _Give the physiology of the organs of hearing._ 963. What is hearing? Are the precise functions of the different parts of the ear known? 964. What is the function of the external ear? 965. Of the membrana tympani? What observation in reference to this membrane? 966.