[responsivevoice_button voice=”UK English Female” buttontext=”Listen Anatomy of Ear“]
The ear is divided into:
Internal ear or the labyrinth
Fig. The ear and its divisions
1. THE EXTERNAL EAR
The external ear consists of the
Auricle or pinna,
External acoustic canal and
i. AURICLE OR PINNA
Fig. Ear The elevations and depressions on the lateral surface of pinna.
The entire pinna except its lobule and the outer part of external acoustic canal are made up of a framework of a single piece of yellow elastic cartilage covered with skin. The latter is closely adherent to the perichondrium on its lateral surface while it is slightly loose on the medial cranial surface. The various elevations and depressions seen on the lateral surface of pinna are shown in Figure [Fig. Ear The elevations and depressions on the lateral surface of pinna.]
Fig. Ear The auricular cartilage.
There is no cartilage between the tragus and crus of the helix, and this area is called incisura terminalis (Fig. Ear The auricular cartilage.). An incision made in this area will not cut through the cartilage and is used for endaural approach in surgery of the external auditory canal or the mastoid. Pinna is also the source of several graft materials for the surgeon. Cartilage from the tragus, perichondrium from the tragus or concha and fat from the lobule are frequently used for reconstructive surgery of the middle ear. The conchal cartilage has also been used to correct the depressed nasal bridge while the composite grafts of the skin and cartilage from the pinna are sometimes used for repair of defects of nasal ala.
ii. EXTERNAL ACOUSTIC (AUDITORY) CANAL
It extends from the bottom of the concha to the tympanic membrane and measures about 24 mm along its posterior wall. It is not a straight tube; its outer part is directed upwards, backwards and medially while its inner part is directed downwards, forwards and medially. Therefore, to see the tympanic membrane, the pinna has to be pulled upwards, backwards and laterally so as to bring the two parts in alignment.
The canal is divided into two parts:
It forms outer one-third (8 mm) of the canal. Cartilage is a continuation of the cartilage which forms the framework of the pinna. It has two deficiencies—the “fissures of Santorini” in this part of the cartilage and through them the parotid or superficial mastoid infections can appear in the canal or vice versa. The skin covering the cartilaginous canal is thick and contains ceruminous and pilosebaceous glands which secrete wax. Hair is only confined to the outer canal and therefore furuncles (staphylococcal infection of hair follicles) are seen only in the outer onethird of the canal.
It forms inner two-thirds (16 mm). Skin lining the bony canal is thin and continuous over the tympanic membrane. It is devoid of hair and ceruminous glands. About 6 mm lateral to tympanic membrane, the bony meatus presents a narrowing called isthmus. Foreign bodies, lodged medial to the isthmus, get impacted, and are difficult to remove. Anteroinferior part of the deep meatus, beyond the isthmus, presents a recess called anterior recess, which acts as a cesspool for discharge and debris in cases of external and middle ear infections (Figure 1.2). Anteroinferior part of the bony canal may present a deficiency (foramen of Huschke) in children up to the age of four or sometimes in adults, permitting infections to and from the parotid.
iii. TYMPANIC MEMBRANE OR THE DRUMHEAD
It forms the partition between the external acoustic canal and the middle ear. It is obliquely set and as a result, its posterosuperior part is more lateral than its anteroinferior part. It is 9–10 mm tall, 8–9 mm wide and 0.1 mm thick. Tympanic membrane can be divided into two parts:
PARS FLACCIDA (SHRAPNELL’S MEMBRANE)
a. PARS TENSA
It forms most of tympanic membrane. Its periphery is thickened to form a fibrocartilaginous ring called annulus tympanicus, which fits in the tympanic sulcus. The central part of pars tensa is tented inwards at the level of the tip of malleus and is called umbo. A bright cone of light can be seen radiating from the tip of malleus to the periphery in the anteroinferior quadrant (Figures 1.3 and 1.4).
b. PARS FLACCIDA (SHRAPNELL’S MEMBRANE)
This is situated above the lateral process of malleus between the notch of Rivinus and the anterior and posterior malleal folds (earlier called malleolar folds). It is not so taut and may appear slightly pinkish. Various landmarks seen on the lateral surface of tympanic membrane are shown in Figure 1.4.
LAYERS OF TYMPANIC MEMBRANE
Tympanic membrane consists of three layers:
Outer epithelial layer
which is continuous with the skin lining the meatus.
Inner mucosal layer,
which is continuous with the mucosa of the middle ear.
Middle fibrous layer
which encloses the handle of malleus and has three types of fibres—the radial, circular and
parabolic (Figure 1.5).
Fibrous layer in the pars flaccida is thin and not organized into various fibres as in pars tensa.
RELATIONS OF EXTERNAL ACOUSTIC MEATUS
Superiorly: Middle cranial fossa Triangular fossa Helix Spine of helix Tail of helix Antitragus
Posteriorly: Mastoid air cells and the facial nerve
Inferiorly: Parotid gland
Anteriorly: Temporomandibular joint
Posterosuperior part of deeper canal near the tympanic membrane is related to the mastoid antrum. “Sagging” of this area may be noticed in acute mastoiditis.
NERVE SUPPLY OF THE EXTERNAL EAR
1. Greater auricular nearve (C2,3) supplies most of the
medial surface of pinna and only posterior part of the
lateral surface (Figure 1.6).
2. Lesser occipital (C2)supplies upper part of medial surface.