Equine Special Senses - Horse Anatomy

Ear

The ear is a paired sensory organ, that is involved in both hearing and balance. For this reason, the ear is known as the vestibulocochlear organ. Sound waves that are transmitted into the ear provide a mechanical stimulus. These mechanical stimuli are then transferred into electrical signals by the cochlea. Neuroreceptors in the ear allow the horse to gain a perception of position and movement. Anatomically, the ear can be looked at in three parts:

1. Outer ear - pinna and auditory canal

2. Middle ear - contains the malleus, incus and stapes bones - known as the ossicles

3. Inner ear - contains the membranous and bony labyrinths, and the cochlea

Outer Ear

Outer Ear - Copyright David Bainbridge

The pinna (or auricle) is defined as the outer projecting part of the ear, that is the part of the ear that can be seen. It consists of the auricular cartilage, surrounded by skin, which allows for flexibility and elasticity. The auricular cartilage connects to scutiform cartilage, which itself connects to annular cartilage, which allows for articulation. These cartilages fit into the bony passage of the ear canal - also called the external auditory meatus, which leads to the tympanic membrane. This is the deepest boundary of the outer ear. Muscles around the base of the ear that are attached to the skull allow movement of the pinna, so the ear can be directed to the source of sound:

Muscle	Origin	Innervation	Function
Preauricular	Deep temporal fascia	Auriculopalpebral branch of facial nerve (cranial nerve VII)	Moves the ear cranio-laterally, so the pinna is facing forwards
Ventroauricular	Laryngeal fascia	Retroauricular branch of facial nerve (cranial nerve VII)	Moves the ear laterally
Postauricular	Medial cervical raphe	Retroauricular branch of facial nerve (cranial nerve VII)	Moves the ear caudo-laterally, so the pinna is facing backwards

Middle Ear

Middle Ear - Copyright David Bainbridge

The middle ear consists of the tympanic cavity, the auditory ossicles and the eustachian tube. The boundary between the middle and inner ear is the oval window. The auditory ossicles are attached to the wall of the tympanic cavity by many ligaments and mucosal folds. The tympanic cavity is located within the petrous temporal bone, and can be divided into dorsal, middle and ventral parts:

• Dorsal: contains the auricular ossicle
• Middle: contians the tympanic membrane within its lateral wall, and opens rostrally into the nasopharynx via the eustachian tube
• Ventral: the tympanic bulla - a thin-walled, bulbous expansion of the temporal bone, which houses an extension of the tympanic cavity

The oval window is positioned rostrodorsally, to which one end of the stapes is attached by an annular ligament. It functions to connect the tympanic cavity with the inner ear. The round window is positioned more caudally, and leads to the cavity of the cochlea.

Sound vibrations are transmitted from the tympanic membrane, across the tympanic cavity, via the ossicles (malleus, incus, then stapes). The ossicles, as well as transmitting sound vibrations from the tympanic membrane, also magnify the vibrations by about 20 times. This is necessary for initiating waves in the endolymph of the cochlea. The magnification is achieved by the action of two muscles that are attached to the ossicles, and which act as antagonists of each other. These two muscles are the tensor tympani muscle and the stapedius muscle. The tensor tympani muscle originates within the tympanic cavity, and inserts on the malleus. The contraction of this muscle creates tension of the ossicles, and therefore also of the tympanic membrane, all of which results in greater sensitivity. The stapedius muscle originates from the wall of the tympanic cavity, and inserts on the stapes. Contraction of this muscle pulls the end of the stapes away from the oval window, thereby reducing the fource of the transmission of sound vibrations.

The eustachian tube connects the tympanic cavity to the nasopharynx, which mark the beginning and end of the eustachian tube, respectively. The eustachian tube functions to equalise pressure on either side of the tympanic cavity, by opening while yawning or swallowing, for example. In the horse, the guttural pouch is a paired diverticulum of the eustacian tube that is unique to this species. This is of clinical importance.

Central Auditory Pathways

===Vestibular Receptors and Balance