Anatomy of the inner, middle and outer ear

The structure of the human ear is divided into three anatomical sections – the inner ear, middle ear and outer ear. The anatomy of this unique part of human physiology makes hearing and balance possible.

Anatomy of the outer ear

The outer ear consists of the visible part of the ear, called the pinna, and the ear canal. The pinna is also sometimes called the auricle, though many reserve that term for the upper portion of the visible ear. The lower portion is often referred to as the ear lobe. The auricle contains elastic cartilage, called the auricular cartilage. It lacks blood vessels and nerve endings. As such it is a permanent cartilage and does not regrow or replace itself – damage is almost always permanent.

The ear canal is a tunnel made of skin and bone, called the external acoustic meatus, which enters the skull, leading to the eardrum. Glands in the outer third of the ear canal produce cerumen, or earwax. It is a moisturizing, cleansing, antibacterial agent that helps keep the path clear for sound waves and prevent infection. Under normal circumstances the wax naturally leaves the ear due to the actions of the muscles of the jaw and mouth (including the levator veli palatini).

Anatomy of the middle ear

The middle ear is also known as the tympanic cavity. It is a pressurized, membrane-lined, air-filled cavity separating the inner ear from the external environment. The pressure is maintained in each ear by a eustachian (or pharyngotympanic) tube. The eardrum is a tympanic membrane separates the outer and middle ear and is held in place by a ring of cartilage (see a diagram for relative location of parts of the middle ear). The eardrum is roughly one-tenth of a millimeter thick and a third of an inch in diameter and consists of three layers of tissue: outer cutaneous, middle fibrous, and inner mucous membrane.

Behind the membrane are three tiny bones, or ossicles, that create the ossicular chain – the hammer, anvil and stirrup when named by their shapes, or more scientifically as the malleus, incus and stapes, respectively. These are the tiniest bones in the human body and are delicate enough to transmit vibrations from the tympanic membrane to the inner ear.

The malleus

The malleus bone is approximately 8 mm in length in adults and consists of a lateral and anterior process when considering it horizontally, and a head, neck and manubrium (shaped somewhat like a hammer) when considering it vertically. The malleus connects to the incus bone via a facet on the head. Attached to the malleus is the tensor tympani muscle, which allows regulation of loud noises to prevent eardrum damage, a process called the the acoustic reflex.

The incus

The incus bone also has a facet where it contacts the malleus. This bone consists of a short crus and long crus, with a curved lenticular process at the end of the long crus where it connects with the stapus bone. The center of the incus bone is referred to as the body.

The stapes

The stapes is the smallest bone at less than one-third of a centimeter in length. This ossicle consists of a head where it makes contact with the incus bone, a neck that divides into the inferior and superior (or sometimes referred to as anterior and posterior) crus (giving it the stirrup or horseshoe shape), and a flat base that makes contact with the oval window. The stapes is stabilized by the stapedius muscle.

Anatomy of the inner ear

The inner ear is a fluid-filled structure containing the auditory and vestibular (i.e. balance) systems. The base of the stapes contacts the outer membrane of the inner ear, called the oval window, or fenstra ovalis. Behind this is the vestibule. The cochlea, semicircular canals, utricle and saccule branch off from this structure.

The auditory system

The cochlea is a spiral-shaped structure divided into three zones. The membranous labyrinth (scala media) is filled with endolymph, which consists of high potassium concentrations, and the bony labyrinth (scala vestibuli and scala tympani) contains perilymph, which is similar in composition to cerebrospinal fluid. The round window in the upper portion of the cochlea is a membrane that acts as a pressure release when sound waves push the oval window. Inside the cochlea is the organ of Corti, which contains the inner and outer hair cells. These hair cells are attached to nerve endings that convey sound information to the brain via the vestibulocochlear nerve (cochlear nerve VIII, also known as the auditory nerve).

The vestibular system

The utricle and saccule are the otolith organs, where nerve endings enter the vestibular system, and are connected via the membranous labyrinth to the semicircular canals. Each inner ear has three semicircular canals lined with cilia and endolymph, with each semicircular canal providing a different directional sensor: horizontal, posterior and superior. The information regarding movement is also sent via cranial nerve VIII.

The structure of the ear

Though the ear is actually made of three different sets of structures, each communicates seamlessly to allow sound waves to reach nerve endings for interpretation by the brain. The placement of the inner ear in the head also gives it a good position for recognizing movement and maintaining balance. For more information about the anatomical terminology and relative locations of structures in the ear, see the three-dimensional model at HealthLine.