CHAPTER IV.

HEARING.*

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 laby

M,

FIG. 17.-Semidiagrammatic section through the right ear (Czermak). concha; G, external auditory meatus; T, tympanic membrane; P, tympanic cavity; o, oval foramen; r, round foramen; R, pharyngeal opening of Eustachian tube; V, vestibule; B, a semicircular canal; S, the cochlea; Vt, scala vestibuli; Pt, scala tympani; A, auditory nerve.

rinth; the latter contains the end-organs of the auditory The external ear consists of the expansion seen on the exterior of the head, called the concha, M, Fig. 17,

nerve.

* In teaching the anatomy of the ear, great assistance will be yielded by the admirable model made by Dr. Auzoux, 56 Rue de Vaugirard, Paris, described in the catalogue of the firm as "No. 21 -Oreille, temporal de 60 cm., nouvelle édition," etc.

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.

"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 stirrupbones. 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 in

ternal 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

FIG. 18.-Mcp, Mc, M, and Mm stand for different parts of the malleus; Jc, Jb, Jl. Jpl, for different parts of the incus or going rapidly down a deep mine, the sudden and

S is the stapes.

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.*

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 bloodvessels 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

*This description is abridged from Martin's 'Human Body'.

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.

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; Fu, 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.

The Membranous Labyrinth.-"The membranous vestibule, lying in the bony, consists of two sacs communicating by a narrow aperture. The posterior is called the utriculus, and into it the membranous semicircular canals open. The anterior, called the sacculus, communicates by a tube with the membranous cochlea. The membranous semicircular canals much resemble the bony, and each has

an ampulla; in the ampulla one side of the membranous

A

tube is closely adherent to its bony protector; at this point nerves enter the former. The relations of the membranous to the bony cochlea are more complicated. section through this part of the auditory apparatus (Fig. 20) shows that its osseous portion consists of a tube wound two and a half times round a central bony axis, the modiolus. From the axis a shelf, the lamina spiralis, projects and partially subdivides the tube, extending farthest across in its lower coils. Attached to the outer edge of this bony plate is the membranous cochlea (scala media), a tube triangular in cross-section and attached by its base to the outer side of the bony cochlear spiral. The spiral lamina and the membranous cochlea thus subdivide the cavity of the bony tube (Fig. 21) into an upper portion, the scala

FIG. 20.-A section through the cochlea in the line of its axis.

FIG. 21.-Section of one coil of the cochlea, magnified. SV, scala vestibuli; R, membrane of Reissner; CC, membranous cochlea (scala media); lls, limbus lamina spiralis; t, tectorial membrane; ST, scala tympani; lso, spiral lamina; Co, rods of Corti; b, basilar membrane.

vestibuli, SV, and a lower, the scala tympani, ST. Between these lie the lamina spiralis (so) and the mem