The term ‘ear’ is often taken to mean the outside of the ear, but the anatomy of the ear extends to the hearing apparatus on the inside of the head too. The ear function is not only to hear, but to enable the brain to determine the position of the head and body and also maintain balance. The anatomy of the ear is split into 3 sections: the outer ear, the middle ear and the inner ear. The inner parts of the ear are in hollow cavities, on each side of the head and placed within the temporal bones of the skull.
The anatomy of the external ear is made up of the pinna or auricle which includes the helix, which is the curved outer rim, the antihelix, which is the inner curve including the ear canal and the outer eardrum or tympanic membrane. The tragus is a small fleshy part which protrudes, and slightly obscures the ear drum while the antitragus is on the opposite side of the opening. The concha is the hollow place in front of the ear canal, which is about 2.5cm long. The ear canal is surrounded by cartilage at first, then surrounded by bone as the ear canal nears the eardrum. This is known as the auditory bulla and formed by the temporal bone tympanic part. Ceruminous and sebaceous glands in the skin around the ear canal produce protective ear wax and the ear canal finishes at the external surface of the eardrum. For some animals, extrinsic and intrinsic muscles enable them to change the direction of the ear. The anatomy of a horse, for example, enables it to move each ear independently. Humans are not usually able to move their ears, although a few can move them slightly. The nerves that supply feeling to the outer ear and skin include the great auricular nerve, auricular nerve, lesser and greater occipital nerves of the cervical plexus and the auriculotemporal nerve.
Blood vessels supplying the external ear include the posterior auricular artery, the anterior auricular artery and the occipital artery.
The middle ear space is filled with air and contains 3 ossicles or tiny bones, which are the malleus (hammer), the incus (anvil) and stapes (stirrup). They are named for the shapes they resemble. Sound waves cause the tympanic membrane to vibrate, the movement is continued by the ossicles which help amplify the sound by 15-20 times, and pass it on to the oval window, which is a thin membrane dividing the middle and inner ears.
The middle ear is connected to the back of the nose and throat (collectively known as nasopharynx) by the narrow Eustachian tube. This enables air to enter the middle ear and mucus to drain from it into the nasopharynx. When a person swallows, the Eustachian tube opens to allow air in to balance air pressure with the other side of the tympanic membrane. If there is a sudden change of air pressure in the environment around, this can cause the pressure in the middle ear to be different, causing the ear drum to bulge or retract and causing temporary problems with hearing. Swallowing to ‘pop’ the ears helps to bring the air pressure back into balance.
The middle ear blood vessels include the mastoid branch of either the occipital or posterior auricular arteries and the deep auricular artery. Arteries which are not as big but help to supply blood, include the middle meningeal artery, the ascending pharyngeal artery, the artery of the pterygoid canal and the internal carotid artery.
The anatomy of the inner ear consists of a bony labyrinth and a membraneous labyrinth, which fit together, separated by a fluid. The inner ear has two main structures: the vestibular system which includes the three semicircular canals at right angles to one another and two small fluid-filled recesses: the saccule and utricle. These help maintain balance. The other structure is the cochlea, which is spiral-shaped and helps to facilitate hearing. The cochlea contains the organ of Corti and is filled with fluid. The organ of Corti contains thousands of specialised sensory hair cells which have projections known as cilla. The sound wave vibrations from the middle ear enable tiny waves to form in the inner ear fluid which makes the cilla vibrate. The vibrations are converted into nerve impulses which the auditory nerve then takes to the brain to be translated into sound. The fenestra cochlea or round window connects the middle ear to the cochlea via a membrane. The semicircular canals also contain hair cells and fluid, although the hair cells here help to detect movement in the fluid. They then send nerve impulses to the brain, which help it understand the position of the head and body and enables the body to balance. The saccule and utricle also work in a similar way to the semicircular canals so that the brain can detect the position of the body relative to gravity so that it can adjust the posture as required.
The vestibulocochlear nerve sends the signals to the brain from the inner ear. The blood vessels for the inner ear include the anterior tympanic branch of the maxillary artery, the posterior auricular artery, the middle meningeal artery and the labyrinthine artery.