Human Ear Anatomy

Human Ear AnatomyAnatomically the ear is divided into three parts, external, middle, and internal. The receptive sensory cells are situated on the walls of the internal ear, a structure of irregular form lying in the substance of the temporal bone one of the bones of the side of the skull; the external ear and middle ear are accessory structures (Fig. 96).

External Ear. - The external ear comprises two portions: (1) an outer expanded part, the auricle or pinna which circles an opening leading into (2) the external auditory meatus.

The meatus is a short tube, about one inch long, open externally, but closed internally by a membranous partition called the tympanic membrane or drum of the ear. The tube presents a double convexity, upwards and backwards.

Middle Ear. - The middle ear or tympanum, is a slit-like cavity between the tympanic membrane and the internal ear.
Opening into it in front is a canal, the Eustachian tube, which leads up from the throat immediately behind the nose, and through which air can enter the tympanum. By means of an opening above, the middle ear communicates with air cells in the substance of the temporal bone. In its interior are three small bones - the auditory ossicles - named the malleus (hammer), incus (anvil), and stapes (stirrup). The chief structure in the outer wall is the tympanic membrane which intervenes between the external auditory meatus and the tympanum; it lies very obliquely forming an angle of about 55 degrees with the floor of the meatus. To the inner surface of the membrane is attached a process (the handle) of the malleus - the tip of the process reaching down to just below the centre of the membrane, which is drawn inwards towards the tympanum. The membrane is thus somewhat funnel-shaped, and the apex of the funnel is known as the umbo.

In the substance of the membrane are two sets of fibres:

(1) fibres radiating outwards from the umbo to the periphery,
(2) circular fibres grouped chiefly towards its circumference

The greater part of the wall between the tympanum and the internal ear is bony, but in it are two small windows. The upper of these is the fenestra ovalis, and into it the foot plate of the stapes is fitted; the other, the fenestra rotunda, is closed by a small membrane. Of the three auditory ossicles (Fig. 97), the malleus is the outermost, the stapes innermost, and the incus is in the centre jointed to both. The handle of the malleus, as already mentioned, is attached to the inner surface of the tympanic membrane; into its upper part is inserted a small muscle, the tensor tympani.

The rounded head of the malleus articulates with the body of the incus; otherwise the bone is tied by ligaments to the tympanic walls. From the body of the incus a long process descends to articulate with the head of the stapes; like the malleus the incus is bound to the walls of the tympanum by ligaments. The stapes is stirrup-shaped; its head articulates with the incus, and just beside this a small muscle, the stapedius, is inserted; the base, which forms the foot plate of the stirrup, is fitted into the fenestra wis.

Internal Ear. - The internal ear consists essentially of a sac containing fluid - endolymph - suspended within another fluid - perilymph. In an early stage of development it is a'simple sac, but later becomes so greatly modified that it is referred to as the membranous labyrinth (Fig. 98). It comprises two bulbs, the utricle and saccule, connected to one another by a Y-shaped duct - the ductus endolymphaticus. From the utricle open three smicircular canals, while the saccule communicates with a spiral tube, the cochlea. The bony cavities in which these are contained, form the bony labyrinth, and the sub­divisions of this are practically those of the membranous labyrinth. Thus there are three bony semicircular canals and a bony cochlea, but the utricle and saccule lie in a common cavity termed the vestibule.

Utricle and Saccule. - On the anterior wall of each of these sacs is a thickening known as the macula acustica (Fig. 99). The thickenings consist of hair cells and supporting cells; the former are provided with long tapering filaments that project into the endolymph within the sacs. Round the hair cells dendrites of the cells of the vestibular nerve are distributed. In contact with the hairs of the maculee are rounded calcareous bodies called otoliths or otoconia.

Semicircular Canals. - Three semicircular canals open into the utricle. Two of these (the superior and the posterior) lie in vertical planes at right angles to one another; the third (external) is horizontal and therefore at right angles to the other two. The vertical planes form approximately angles of 45 degrees with the mesial antero-posterior plane; the canals of opposite sides are therefore arranged in parallel pairs. The two horizontal canals lie in planes parallel to one another; the posterior canal of one side is parallel to the superior of the other (Fig. 106).

One end of each semicircular canal presents a dilatation or ampulla. The non-ampullated ends of the posterior and superior canals are fused, and open by a common opening, so that there are but five openings into the utricle. In each ampulla is a transverse thickening, the crista acustica, which, like the maculae of the utricle and saccule, consists of hair cells and supporting cells. The dendrites of the vestibular neurons terminate round the hair cells.

Membranous Cochlea. - The membranous cochlea is a spirally coiled tube communicating with the saccule by a narrower portion, the canalis reuniens. The spiral tube is attached to the modiolus or central pillar and to the outer wall of the bony cochlea. Three channels are thus recognisable in the complete cochlea: (1) the membranous cochlea containing endolymph; (2) a channel above this, the scala vestibuli and (3) one below it, the scala tympani (Fig. 100). The two latter contain perilymph; they communicate with one another at the apex of the cochlea through a small opening termed the helicotrema, while behind they are continuous with the general perilymph space in which the utricle, saccule, and membranous semicircular canals are suspended. In the outer walls of the bony labyrinth are the two windows already described; the fenestra ovalis opens into the vestibule, the fenestra rotunda into the scala tympani.

The membranous cochlea is triangular on cross-section. Winding round the modiolus is a spiral shelf of bone, the lamina spiralis, and to this the apex of the triangle is attached; the base is applied to the outer wall of the bony canal. The lower side of the membranous cochlea is attached to the margin of the spiral lamina and is known as the basilar membrane; the upper side is attached nearer the root of the lamina and is termed the membrane of Reissner. Both membranes extend across the bony cochlea, and are bound to the outer wall of this by a fibrous structure, the spiral ligament. The basilar membrane exhibits a series of transverse radiating fibres that increase in length from below upwards as the membrane increases in width. In the modiolus the ganglion of the cochlear nerve is situated, and from its cells dendrites extend along the spiral lamina to a peculiar receptive structure on the basilar membrane - the organ of Corti.