The middle ear consists of an air-filled tympanic cavity (cavum tympani) connected with the nasopharynx via the auditory tube (tuba auditiva), and closed to the outside by the tympanic membrane (membrana tympani) at the level of the external acoustic meatus. The tympanic cavity has a small, dorsal epitympanic recess and a large, ventral tympanic bulla (bulla tympanica). The middle portion of the tympanic cavity contains the three auditory ossicles — malleus, incus, and stapes — and the two muscles associated with them — the tensor tympani on the malleus and the stapedius on the stapes.
The eardrum, or tympanic membrane (membrana tympani), covers the entrance to the tympanic cavity and separates the middle ear cavity from the external acoustic meatus. It is a thin, semitransparent, three-layered membrane somewhat oval in shape and concave when viewed externally. The inner epithelium is of pharyngeal pouch origin, the central layer is fibrous connective tissue of the pharyngeal wall, and the outer stratified squamous epithelium is derived from the ectoderm of the first branchial groove.
The tympanic membrane may be divided into two parts: the pars flaccida and the pars tensa. The pars flaccida is a small dorsal triangular portion that lies between the short lateral process of the malleus and the margins of the tympanic incisure. The pars tensa constitutes the remainder of the membrane that attaches peripherally to the fibrocartilaginous anulus. Wakuri et al. (1988) examined the connective tissue layer of the dog tympanum and found outer radial and inner circular fibers in the pars tensa.
The external aspect of the tympanic membrane is somewhat concave, owing to traction on the medial surface by the manubrium of the malleus. The most depressed point, which is opposite the distal end of the manubrium, is termed the umbo membranae tympani. A light-colored streak, stria mallearis, may be seen running dorsocaudally from the umbo toward the pars flaccida when viewed from the external side. This is caused by the manubrium being partly visible through the tympanic membrane along its attachment. The manubrium is embedded in the tunica propria and is covered with the epithelium of the tympanic cavity that lines the membrane. The epithelium on the external surface of the tympanic membrane originates around the site of attachment of the manubrium of the malleus to the membrane. From this site at the umbo of the tympanic membrane, there is continuous mitotic proliferation and migration across the membrane, radiating in all directions. This migration cleanses the membrane of keratinized debris by moving it first to the periphery, and then into the external acoustic meatus, where it builds up and is shed via the external ear canal. When injured or perforated the tympanic membrane is repaired in the same manner. Within the ear canal there are sebaceous glands often associated with hair follicles and deeper tubular glands that produce cerumen, or earwax.
Maher (1988) injected and cleared the tympanic membranes and adnexa of neonatal dogs to study microangiology relative to surgical procedures. He found the structure and arterial supply of the dog tympanic membrane to be very similar to that of the human. There was a dual source of arterial supply: extrinsic sources from the stylomastoid branch of the caudal auricular artery and intrinsic sources from deep auricular and rostral tympanic branches of the maxillary artery. More specifically, the tympanic membrane was supplied centrally by ramifications from the malleus periosteal network and circumferentially by ramifications from the annulus perichondrial network. The origin of the tympanic membrane venous plexus was from an extensive capillary bed that ultimately formed two distinct venous pathways. The principal route extended into the middle ear via the pars flaccida and joined the middle ear venous plexus. Minor routes joined cutaneous veins of the external acoustic meatus at the tympanic membrane junction. The exit portals for veins were not the same as the entrance portals for arteries.