It is important for animals (incidentally all chordates are free moving) to maintain the correct posture and to detect signals in the form of changes in the pattern of sound waves. The appearance of the ear in chordates actually begins from the incorporation of the membranous labryinth in cyclostomes. Further development concerns the addition of ear ossicles in the middle ear by the modification of visceral arches and the progressive development of the membranous labyrinth. The ear is best developed in mammals where all the divisions - external, middle and inner are present with their respective components.
Development of the ear
A small patch of ectoderm on each side of the head in the region of the hind brain thickens to form an auditory placode. This sinks in and thereafter invaginates to form an auditory vesicle/autocyst. It lies embedded in the mesenchyme of the head. The invagination canal connets each auditory vesicle to the outside. In elasmobranchs, this remains open. In other vertebrates, the auditory vesicle remains as a closed sac having no invagination canal. Ductus endolymphaticus / endolymphatic duct arises as a new outgrowth from the auditory vesicle. It has no external opening. In amphibians and some reptiles, the end of this duct expands to form an endolymphatic sac. The auditory vesicle now constricts to form a dorsal utriculus and a ventral sacculus. One/two/three semicircular canals also appear and open into the utriculus at both ends. One end of each canal expands to form an ampulla. Two parallel grooves arise and these two meet and ___ to form a tube. An evagination called lagena arises from the lower part of the sacculus. This appears first in fish. In successive groups, it begins to grow increasingly. The auditory capsule is formed by the mesenchyme. It is first made of cartilage and is later replaced by bone. It encloses the membranous labyrinth.
The middle ear and eustachian tube are derived from the spiracles of fish. The upper part of the hyomandibular arch gives rise to the columella auris of lower tetrapods and the stapes of higher tetrapods. The external opening of the spiracle becomes closed and it fuses with with the skin to form the tympanic membrane. The cavity of the spiracle expands to form the cavity of the middle ear. The middle ear establishes a connection with the pharynx by the Eustachian tube. During the transition of aquatic to terrestrial mode of life, the spiracle becomes a sound conducting device. In mammals, the stapes is joined by two more ear ossciles. The quadrate gets modifies into the incus and the articular gets modified into the malleus. These are joined to each other and also connect the tympanic membrane with the internal ear. The external ear / pinna, in mammals, is formed mainly as an outgrowth of the skin. It is reinforced by an elastic cartilage.
COMPARATIVE ACCOUNT
Cyclostomes and Fish
The middle ear and external ear are absent.
Amphibia (e.g. frog)
The middle ear is visible from outside due to the absence of pinna. It encloses an air-filled tympanic cavity. It is limited internally by the auditory capsule and externally by the tympanic membrane / tympanum. This is visible as a dark tightly stretched patch of skin. The cavity of the middle ear communicates with the pharynx by a slender, narrow passage called the eustachian tube running downward and opening into the buccopharyngeal cavity just near the angle of the mouth. The tympanic membrane is tightly stretched over a ring of cartilage called the annulus tympanicus. It is modified skin. A club-shaped rod, the columella auris, touches the centre of the tympanum and is extended across the tympanic cavity to a cartilagenous, small nodule known as the stapedial plate, which is fised with a hole in the auditory capsule called the fenestra ovalis. Structurally, the columella auris is made up of bone as well as cartilage. A ring-like bone, the operculum is present in the fenestra ovalis.
Reptiles (e.g. lizard)
The middle ear is represented by an air-filled cavity called the tympanic cavity. It is derived from the firdt pharyngeal pouch. The external limit of the tympanic cavity is formed by the tympanic membrane, whereas, its internal boundary is limited by the auditory capsule. A narrow passage called the eustachean tube / pharyngo-tympanic tube extends downward and inward to open into the posterior part of the pharynx. The eustachian tube of reptiles is narrower and longer than that of amphibians. A single, rod-like ear ossicle, the columella auris stretches across the tympanic cavity. It is derived from the hyomandibular of the hyoid arch. This is made of
(i) An inner body - stapes / stapedial bone
(ii) An outer cartilagenous extrastapedial / extracolumellar cartilage.
The stapes fits into a small membrane covered aperture called the fenestra ovalis located in the outer wall of the auditory capsule. The extracolumellar cartilage is attached to the inner surface of the tympanum. One more membrane covered aperture is present in the outer wall of the auditory capsule. It is called the fenestra rotunda.
Variations
The tympanum, eustachian tube and the tympanic cavity are totally absent in snakes. Yet, snakes recieve sound vibrations with the help of the columella auris which is attacched at its outer end to the quadrate.
In crocodiles, the two eustachian tubes open into the ___________
In turtles, the tympanic membrane is thin and delicate, while it is thick and covered with skin in the terrestrial form. In crocodiles, a movable integumentary fold covers and protects the depressed tympanic membrane.
Aves (e.g. pigeon)
The middle ear is an air-filled cavity. The tympanic cavity is limited externally by th tympanum and internally by the auditory capsule. A narrow eustachian / pharyngo-tympanic tube arises from the lower medial part of the tympanic cavity and extends downward and inward. The two eustachian tubes join to finally open into the roof of the pharynx by a common aperture. The columella auris extends across the tympanic cavity and can be easily distinguished into four parts - an inner, disc-like bony stapes and an outer three-layered cartilagenous extracolumella. The bony stapes fits into a membrane- covered aperture in the outer wall of the auditory capsule called the fenestra ovalis. The extracolumella is attached to the inner surface of the tympanum. There is present an additional circular aperture below the fenestra ovalis ____________. It is also membrane covered.
Mammal (e.g. rabbit)
The middle ear is represented by an irregular air-filled space called the tympanic cavity, enclosed in the tympanic bulla. It is lined by a mucus membrane. It communicates with the pharynx by a passage called the eustachian tube which extends downward and inward. The periotic bone forms the inner wall of the tympanic cavity. This wall bears two apertures - fenestra ovalis / oval window and the fenestra rotunda / round window, both covered with a thin membrane of connective tissue.
An important feature of the middle ear is the presencce of three ear ossicles - malleus, incus and stapes. These are movably articulated with each other extending from the point of the fenestra ovalis to the periotic bone. The malleus is the outermost, hammer-shaped ossicle and is attached to the inner surface of the tympanum. The middle ossicle, incus, is anvil-shaped. It articulates with the malleus by a synovial joint and with the stapes by a ball-and-sockect joint. The inner moer is the stapes. These three ossicles function as a system of levers.
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