Changes

Anatomically, the ear can be looked at in three parts:

Anatomically, the ear can be looked at in three parts:

1. '''Outer ear''' - pinna and auditory canal

1. '''Outer ear''' - pinna and auditory canal down to the level of the tympanic membrane

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

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

===Outer Ear===

===Outer Ear===

[[Image:Outer Ear.jpg|thumb|right|150px|Outer Ear - Copyright David Bainbridge]]

[[Image:Outer Ear.jpg|thumb|right|150px|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.

The pinna consists of the '''auricular [[Cartilage - Anatomy & Physiology#Structure and Function of Cartilage|cartilage]]''', and skin which allows for flexibility and elasticity. The auricular cartilage is flared distally into a flattened cone shape, covered by skin on both sides - more tightly on the medial or concave side than the convex side.  The [[Hair - Anatomy & Physiology|hair]] covering on the convex side is usually similar to the rest of the body but the hair covering on the concave or medial aspect is variable.  There are also irregularities on the concave surface with ridges and prominences, the '''medial and lateral crus of the helix''' on the medial aspect of the opening of the external ear canal, opposite the rectangular '''tragus''' on the lateral aspect.

 

Proximally, the auricular cartilage is rolled into a funnel shape, known as the '''concha'''.  The '''scutiform''' cartilage lies rostromedially in the lower ear canal and provides support. The '''annular''' cartilage lies between the proximal horizontal ear canal and the bony '''acoustic process''' of the [[Skull and Facial Muscles - Anatomy & Physiology#Temporal Bone (os temporale)|typmanic bulla]], connected by ligaments.  The vertical ear canal lies in a rostroventral orientation before bending medially to become the horizontal canal.

The cartilage and bony process of the external ear and the tympanic membrane are covered by skin.  The skin of the normal ear has a thin stratified keratinising epidermis and a thin dermis containing adnexal structures – [[Hair - Anatomy & Physiology|hair follicles]], [[Skin - Anatomy & Physiology#Glands|sebaceous and apocrine glands]].  The density of hair follicles on the concave pinna is variable, as is the density and distribution of hair follicles in the ear canal itself.  Some species are devoid of hair in the lower (proximal) ear canal (eg horse); hair follicle density in the dog is variable – usually simple but in some breeds compound hair follicles.

The cartilage and bony process of the external ear and the tympanic membrane are covered by skin.  The skin of the normal ear has a thin stratified keratinising epidermis and a thin dermis containing adnexal structures – [[Hair - Anatomy & Physiology|hair follicles]], [[Skin - Anatomy & Physiology#Glands|sebaceous and apocrine glands]].  The density of hair follicles on the concave pinna is variable, as is the density and distribution of hair follicles in the ear canal itself.  Some species are devoid of hair in the lower (proximal) ear canal (eg horse); hair follicle density in the dog is variable – usually simple but in some breeds compound hair follicles.

|}

|}

[[image: Canine Auricular Cartilages.jpg|thumb|250px|right|The auricular cartilages of the left canine ear. Image by Rachael Wallace]]

[[image: Canine Auricular Cartilages.jpg|thumb|250px|right|The auricular cartilages of the left canine ear. Image by Rachael Wallace]]

*The tympanic cavity is located within the petrous temporal bone, and can be divided into dorsal, middle and ventral parts:

*The tympanic cavity is located within the petrous temporal bone, and can be divided into dorsal, middle and ventral parts:

- ''Dorsal'': contains the auditory ossicles

- ''Dorsal'': or '''epitympanum''', conatining 2 of the auditory ossicles – the '''malleus''' and '''incus'''

- ''Middle'': contians the tympanic membrane within its lateral wall, and opens rostrally into the nasopharynx via the eustachian tube

- ''Middle'': or '''mesotympanum''' bounded by the tympanic membrane laterally and containing the third auditory ossicle, '''stapes''', attached to the oval window. It 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

- ''Ventral'': or '''hypotympanum''', or fundic cavity, which is the largest compartment. It is housed by the tympanic bulla which is a thin-walled, bulbous expansion of the temporal bone.

*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 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.

*The round window is positioned more caudally, and leads to the cavity of the cochlea. The '''round window''' is on the caudomedial aspect of the mesotympanum and the opening of the auditory '''(Eustachian)''' tube opens on rostromedial aspect of the mesotympanum.

*Sound vibrations are transmitted from the tympanic membrane, across the tympanic cavity, via the ossicles (malleus, incus, then stapes).

*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 force of the transmission of sound vibrations.

*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 force of the transmission of sound vibrations.

*The eustachian tube connects the tympanic cavity to the nasopharynx.

*The eustachian tube connects the tympanic cavity to the nasopharynx.

*The eustachian tube functions to equalise pressure on either side of the tympanic cavity, by opening while yawning or swallowing, for example.

*The eustachian tube functions to equalise pressure on either side of the tympanic cavity, by opening while yawning or swallowing, for example.

[[image: Canine Ear Canal.jpg|thumb|250px|right|The Canine Middle Ear]]

[[image: Canine Ear Canal.jpg|thumb|250px|right|The Canine Middle Ear]]

It has 3 compartments,  the smallest dorsal or '''epitympanum''', conatining 2 of the auditory ossicles – the '''malleus''' and '''incus'''; the middle or '''mesotympanum''' bounded by the tympanic membrane laterally and containing the third auditory ossicle, '''stapes''', attached to the oval window and the ventral, '''hypotympamum''', or fundic cavity, which is the largest compartment.  The auditory ossicles are supported by ligaments and muscles, which alter their position and influences the tension of the tympanic membrane.  The ratio of the malleus:incus in dogs and cats is 2-3 times that of man, and may explain the increased acuity of hearing.  The '''round window''' is on the caudomedial aspect of the mesotympanum and the opening of the auditory '''(Eustachian)''' tube opens on rostromedial aspect of the mesotympanum.  Opposite the lateral tympanic membrane on the medial aspect of the cavity is a bony promontory.  Associated structures close to or in channels in the wall of the tympanic cavity (bulla) are the '''facial nerve''', '''vagus nerve''' and branches of the '''carotid and lingual arteries'''.  Post-ganglionic fibres of the '''cervical sympathetic trunk''' run in the region of the dorsomedial wall of the tympanic cavity.

The auditory ossicles are supported by ligaments and muscles, which alter their position and influences the tension of the tympanic membrane.  The ratio of the malleus:incus in dogs and cats is 2-3 times that of man, and may explain the increased acuity of hearing.  Opposite the lateral tympanic membrane on the medial aspect of the cavity is a bony promontory.  Associated structures close to or in channels in the wall of the tympanic cavity (bulla) are the '''facial nerve''', '''vagus nerve''' and branches of the '''carotid and lingual arteries'''.  Post-ganglionic fibres of the '''cervical sympathetic trunk''' run in the region of the dorsomedial wall of the tympanic cavity.

The feline middle ear has an incomplete '''bony septum''' dividing the ventral chamber into a large ventromedial and small dorsolateral chamber, communicating caudomedially.

The feline middle ear has an incomplete '''bony septum''' dividing the ventral chamber into a large ventromedial and small dorsolateral chamber, communicating caudomedially.

- ''Cochlea'': forms a spiral around a central hollow core of bone, called the modiolus,which contains the cochlear nerve. The spiral lamina projects into the spiral canal, partially bisecting the lumen into two parts, which are called the scala tympani and the scala vestibuli. The scala media (the cochlear duct) is inbetween these two parts.

- ''Cochlea'': forms a spiral around a central hollow core of bone, called the modiolus,which contains the cochlear nerve. The spiral lamina projects into the spiral canal, partially bisecting the lumen into two parts, which are called the scala tympani and the scala vestibuli. The scala media (the cochlear duct) is inbetween these two parts.

==Function - Hearing==

==Function - Hearing==