Written by Anjani Mishra
Ear
The ear is the organ of hearing and equilibration and
may be considered as vestibulocochlear organ. It is divisible into three parts;
1. External
2. Middle
(tympanic cavity)
3. Internal ear
1. External ear: (auris externa)- the external ear consists of the auricle or pinna, and external acoustic meatus. It conducts sound to the middle ear and serves to protect the middle and inner ear. The auricle projects from the head and serves to collect the vibrations of the air by which sound is produced; in domestic animals, unlike man, the auricle can be turned in different positions without turning the head by means of ear muscles. The external acoustic meatus leads inward from the bottom of the auricle and conduct vibrations to the tympanic cavity.
2. Middle ear: (auris media)- the middle ear is an irregular space within the temporal bone that is filled with air, which is conveyed to it from the nasal part of the pharynx through the auditory tube. It contains the ossicles (malleus, incus, stapes) a chain of three tiny movable bones which form a bridge and serve to convey the vibration from the external ear communicated to the tympanic membrane across the cavity to the internal ear.
The malleus, so named from its fancied resemblance to a hammer, is attached to the tympanic membrane and consists of head, neck and three processes- manubrium and rostral and lateral processes
The incus has received its name from its resemblance to an anvil, but it is more like a premolar tooth with two roots, which differ in length and are widely separated from each other. The incus is located between and connect to the malleus and stapes by delicate articulations. It consists of a body and two crura- short and long.
The stapes, so called from its resemblance to a stirrup, consists of a head, neck, two crura and a base, and is attached to the vestibular window.
Ø The ossicles are connected with the walls of the tympanic cavity by ligaments.
Ø The mucous membrane of the tympanic cavity is continuous with that of the pharynx through the auditory tube. It invests the auditory ossicles and the muscles and nerves contained in the tympanic cavity and forms the medial layer of the tympanic membrane and the lateral layer of the secondary tympanic membrane and is reflected into the tympanic antrum and cells.
Ø It forms several folds which extend from the walls of the tympanic cavity to the ossicles. These folds separate off pouch-like cavities and give the interior of the tympanum a somewhat honeycombed appearance.
3. Internal ear: (auris interna)- the internal ear receives the ultimate distribution of the acoustic nerve. It is called the labyrinth because of the complexity of its shape; it consists of two parts;
A. The
osseous labyrinth, a series of cavities within the petrous part of the
temporal
bone, and
B. The
membranous labyrinth, a series of communicating membranous sac and
ducts contained within the bony cavities.
A. The
osseous labyrinth consists of three parts;
a. Semicircular
canals
b. The
vestibule
c. Cochlea
a. Semicircular canals-
These are three in number, as follows;
i) Superior/anterior,
ii) Posterior,
&
iii) Lateral
Ø They
are curved tubes placed at right angles to each other.
Ø These
canals are about 25-30 mm long and join with the vestibule by five
openings.
Ø The
anterior and posterior canals have a common opening. The terminal parts of
the tube at their junctions with the vestibule are dilated to form ampullae.
b. The vestibule-
Ø It
is a bony cavity placed medial to the tympanic cavity. It is continuous with
the cochlea in front and semicircular canals behind.
Ø The
oval window is situated at the lateral wall. The medial wall is perforated for
the passage of the filaments of vestibulocochlear nerve and aqueduct of
vestibule.
Ø The
aqueduct is extended through the petrous part of temporal bone.
Ø Three
semicircular canals are attached by five orifices at the posterior part of the
vestibule.
Ø The
cavity of the vestibule lodges the utricle and saccule of membranous labyrinth.
Ø These are cavities hollowed out of the substance of the bone and lined by peritoneum. They contain a clear fluid, the perilymph in which the membranous labyrinth is suspended.
c. Cochlea-
It is a bony spiral canal winds two and a
half turn around a osseous modiolus and has a appearance of a shell of a snail.
B.
The
membranous labyrinth
Ø The
membranous labyrinth is lodged within the bony cavities and has the same
general form as these; it is however, considerably smaller and is partly
separated from the bony walls by a fluid, the perilymph.
Ø In
certain places it is fixed to the walls of the cavity. The membranous labyrinth
contains fluid, the endolymph.
Ø Ramifications
of the acoustic nerve are distributed on its walls. Within the osseous
vestibule the membranous labyrinth does not quite preserve the form of the bony
cavity, but consists of two membranous sacs, the utricle and saccule.
Organ
of Corti
Ø This
organ is situated on the basilar membrane. It contains hair cells, tunnel of corti,
tectorial membrane and spinal ganglion.
Ø The
organ is winded spirally throughout the length of the membranous cochlear duct.
Ø The
hair
cells are placed on the basilar membrane in an outer and an inner row.
The inner cells are arranged in a single line and the outer cells are arranged
in four lines.
Ø The
outer and inner rods of corti unite at the upper ends and thereby enclose the tunnel
of corti.
Ø The tectorial membrane is attached to the outer border of spiral lamina. It ia made up of gelatinous substance and overhangs the hair cells.
The spiral ganglions are situated on the bony spiral lamina. Its bipolar cells receive fibers from the bases of inner and outer hair cells. The axons of these spiral ganglionic nerve cells constitute the cochlear division of the auditory nerve.
Facebook Veterinary group
Post a Comment