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| |    Understanding Ear Infections: Anatomy of the Ear by Jerome O. Klein, M.D. reviewed by Lynn Cates, M.D., F.A.A.P. and Laura Jana, M.D., F.A.A.P. Knowing a little about the anatomy of the ear is useful in understanding ear infections. The ear is divided into three main areas: the external ear canal and surrounding areas, the middle ear, and the inner ear. The external ear The external ear canal is about an inch long. It is narrow and tortuous and goes from the outside part of the ear you can see, all the way in to the eardrum (or tympanic membrane). The outer half of the ear canal is surrounded and supported by cartilage, while the inner half tunnels through a portion of the skull known as the temporal bone. A narrowing between these two halves of the canal serves as a barrier to prevent wax and foreign bodies from going too far down the canal and into the area near the eardrum. The skin around the outer ear is continuous with the skin covering the walls of the canal. The middle ear The eardrum serves as the outer wall of the middle ear, separating it from the ear canal. The middle ear is part of a mucous-lined system that is continuous from the nose through the eustachian tube (a double-horn-shaped organ with a narrow center) and the middle ear space. The middle ear space contains the small bones (or ossicles) that serve to transmit sound. The space is a flattened box that is about half an inch high, slightly less than half an inch wide, and less than one-quarter of an inch deep. This narrow box is bounded on the outside by the eardrum; on the inside by bone and membranous windows that allow sound to be transmitted to the inner ear, behind by the mastoid sinus; and in front by a wall that serves as the entry for the eustachian tube. The anatomy of the eustachian tube changes over the first few years of life: In infancy it is shorter, relatively wider and horizontal; in children older than two years it elongates, narrows, and becomes more oblique. The shape and alignment of the eustachian tube in early infancy is partly responsible for the higher incidence of ear infections in infancy. The inner ear The inner ear contains small, coiled organs including the semicircular canals and cochlea that house the hearing nerve and the organs responsible for regulating balance.
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