The Ear

The ear (organum vestibulocochleare [auris]) evolved as an organ of balance and hearing in vertebrates. The most primitive part is the inner ear (auris interna), which in all vertebrates consists of a membranous labyrinth within a bony labyrinth and functions for both balance and hearing. Fish have only an inner ear, whereas amphibians and reptiles developed an additional chamber, the middle ear (auris media), formed by a tympanic cavity (cavum tympani) that is an extension of the pharynx. The tympanic cavity connects with the pharynx via the auditory tube, (formerly eustachian or pharyngotympanic tube), which is closed to the outside by a tympanic membrane, or eardrum. A portion of the hyomandibular bone from the second branchial arch develops into a soundconducting ossicle, the columella, which transmits vibrations from the tympanic membrane, across the air-filled tympanic cavity, to the inner ear. In mammals two more bones, the malleus and incus, are added to the middle ear, and the columella becomes a stapes, making a chain of three bones from the tympanic membrane to the inner ear. Although lizards and birds have an external ear (auris externa) it consists only of an auditory meatus and a short canal. To protect the entrance to the canal and help direct sound into it, lizards sometimes have specialized scales, and birds have specialized feathers and a cutaneous muscle for pulling a skin flap across the opening. Only mammals (except cetaceans and a few others) have a well-defined external ear formed by a cartilaginous auricle (auricula) or pinna covered by skin and moved by muscles.

The three major components of the dog ear will be considered here in developmental sequence, which is similar to their phylogenetic evolution: (1) the inner ear, (2) the middle ear, and (3) the external ear.

The receptor for special proprioception, the vestibular system, develops in conjunction with the receptor for the auditory system (special somatic afferent system). They are derived from ectoderm but are contained in a mesodermally derived structure. Together these receptors are the components of the inner ear. The ectodermal component arises as a proliferation of ectodermal epithelial cells on the surface of the embryo adjacent to the developing rhombencephalon. This structure is the otic placode, which subsequently invaginates to form an otic pit and otic vesicle (otocyst) that breaks away from its attachment to the surface ectoderm. This saccular structure undergoes extensive modification of its shape but always retains its fluid-filled lumen (endolymph) and surrounding thin epithelial wall as it becomes the membranous labyrinth of the inner ear. Special modifications of its epithelial surface at predetermined sites form the receptor organs for the vestibular and auditory systems.

Corresponding developmental modifications occur in the surrounding paraxial mesoderm to provide a supporting capsule for the membranous labyrinth. This fluid-filled (perilymph) ossified structure is the bony labyrinth contained within the developing petrous portion of the temporal bone.

These membranous and bony labyrinths are formed adjacent to the first and second branchial arches and their corresponding first pharyngeal pouch and first branchial groove. The first branchial groove gives rise to the external ear canal. The first pharyngeal pouch forms the auditory tube and the mucosa of the middle-ear cavity. The intervening tissue forms the tympanum. The ear ossicles are derived from the neural crest of branchial arches 1 (malleus and incus) and 2 (stapes). These ossicles become components of the middle ear associated laterally with the tympanum (malleus) and medially with the vestibular window of the bony labyrinth of the inner ear (stapes).

The Inner Ear

The Middle Ear

The External Ear