Sense organs allow animals to monitor their internal and external environments by detecting stimuli via specialized receptor cells. There are five main types of receptors that respond to mechanical, thermal, light, chemical, and painful stimuli. Receptors are classified as extroceptors on the surface, visceroceptors for internal organs, or proprioceptors for body position. Special sense organs like the eye, ear, nose and tongue contain concentrated sensory cells and detect vision, sound, smell and taste respectively. The eye contains three tunics - fibrous, vascular and nervous - that house light-sensitive retinal cells. Accessory structures like the eyelids and muscles help move and protect the eyeball.

1. SENSE ORGANS
Sense organs are
 Extensions of the CNS that allow it to monitor what is
going on inside and outside the animal body.
 Made up of receptor cells by which individual animal is
aware of changes in internal and external environments.
 The internal and external environmental changes that are
able to initiate nerve impulse is known as stimuli.
 Sensory experiences begin at receptors,
 Receptors are specialized cells or nerve endings that
detect a particular aspect of the internal and external
environmental changes)

2.  When the sensory receptors are triggered by stimulus, they
generate a nerve impulse that travels to the CNS and is
interpreted as a particular sensation.
Receptors: are classified based on their nature (sensitivity)
of stimulus and their location
 Based on the nature of stimulus, there are five general
types of receptors:
1. Mechanoreceptors– respond to physical/ mechanical
stimuli
2. Thermoreceptors– respond for both heat and cold
3. Photoreceptors – respond to light
4. Chemoreceptors – respond to chemical changes – eg-
taste, smell
5. Nociceptors – respond to injurious (noxious)(pain)
stimuli

3. • Depending on the location, there are three types of
receptors:
1. Extroceptors – located near the surface of the body.
• E.g. Cones and rods in the retina of eyes–
photoreceptors (light).
• Olfactory receptors in nasal cavity– Chemoreceptor
(chemicals).
2. Visceroceptors – produce sensation arising from the
viscera (internal organs)
– Eg. – Internal pain, hunger, thirst, fatigue or nausea
3. Proprioceptors – receptors that are relayed on the
information about body position, equilibrium and
movement.
– Eg. – Located in the inner ear, joints, tendons and
muscles

4. Cont…
• The sensory receptors of domestic animals are
sensitive to four general types of stimuli
1. Mechanical stimuli E.g.- touch, hearing, balance
2. Thermal stimuli E.g. – Hot and cold
3. Electromagnetic stimuli E.g.- Vision
4. Chemical stimuli E.g. – Taste and Smell

5. Cont…
Sense organs: are divided into two classes
I. Organs of special sense
II. Organs of general senses
I. Organs of Special Senses
• They are organs of concentrated and highly
specialized sensory cells, such as eyes, ears,
olfactory and taste organs
• Mammals possess four special sense organs
Organ of Vision
Organ of Taste
Organ of Hearing and Equilibrium
Organs of Smell

6. Special Senses
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Sense What Is Sensed Type of Stimulus
Smell Odors Chemical
Taste Tastes Chemical
Hearing Sounds Mechanical
Equilibrium Balance and head
position
Mechanical
Vision Light Electromagnetic

7. A. Organ of Vision (The eye)
• The eye is the most highly developed organ of special
senses.
• It receives stimuli in the form of light=> Light is
changed by means of nervous structures (Eg. Retina)
into nerve impulses that can be interpreted as picture by
brain.
• Consists of: - The eye ball
- The accessory ocular structures
Eyelid and conjunctiva
Lacrimal apparatus
Ocular muscle (Bulbar muscles)
Orbital fascia

8. Cont…
• These all structures are housed in the bony cavity
called orbit.
Formed by frontal, zygomatic, lacrimal, maxilla,
sphenoid, palatine and temporal bones.
• The shape and depth of the orbit (bony cavity)
determines the appearance and extent of visual
field of the eye of animals
• The eyes of domestic animals protrude from the
surface of the face than those of primates.
• Their position in the head is related to the animal’s
environment, habits and methods of feeding

9. Cont…
• In general, predator species of animals have
eyes set well forward, increasing binocular
vision, where as those that are hunted carry their
eyes more laterally, and so constantly aware of
large area of their surroundings

10. The eye ball (bulbus occuli)
• The eye ball of domestic animals is nearly
spherical, but with anterior-posterior
compressions in horses and cattle.
• It has three tunics
1. Fibrous tunic – external
2. Vascular tunic – middle
3. Nervous tunic – internal
1. The Fibrous tunic
• It is the external coat of the eye ball, and made
up of dense collagenous tissue, which gives the
eye its shape and stiffness by resisting internal
pressure.
They form a laminated sheet
and surround the partly liquid
and partly gelatinous center

11. Cont…
• This tunic consists of
– Sclera (posterior Opaque part) and
– Cornea (anterior transparent)
• Sclera and cornea meet at corneal-sclera junction
called Limbus.
Sclera
• The opaque posterior part of the fibrous tunic
• It consists of dense collagenous and elastic fibers
• Generally it is white “The white of the eye”
• Posteriorly, it presents a small cribriform area through
which the optic nerve fiber passes
• The sclera is pierced by small cilliary arteries, nerves
and large vorticose veins

12. Cont…
Cornea
• It is the anterior transparent and non-vascular
portion of the fibrous tunic
• It is composed of a special type of dense
connective tissue arranged in a lamellar sheet
• The surface of cornea is very sensitive owing to
the presence of free nerve endings
As a result corneal reflex forms, i.e. closing of
the eye lid when touched occurs
Corneal reflex – is important to monitor the
stage of anesthesia during surgery

15. Cont…
2. Vascular tunic (Uvea)
• The middle layer of the eye ball
• It lies deep to the sclera and is composed of three zones;
the choroids, ciliary body and iris
• It contains blood vessels, intrinsic muscles and nerves
Choroids: - a thin highly vascular layer lining the internal
surface of sclera from the optic nerve to the limbus
– It contains a dense network of blood vessels embedded
in heavily pigmented connective tissue
– In the dorsal part of the interior surface of the posterior
hemisphere (fundus), the choroid forms a variously
colored light-reflecting area, tapetum lucidum, which
makes the shining of animal’s eye when light is directed
towards it (in dark place)
It is absent in swine and man

16. Cont…
Ciliary body: - it is formed near the limbus where the
choroids thicken.
– It is a raised ring with ridges radiating toward the lens in
the center, anteriorly the ring is continued by iris.
– The radial ridges of the ciliary body are ciliary processes
that surround the lens and extend zonular fibers towards
the lens to suspend it around its periphery.
– Smooth ciliary muscle is located between ciliary body and
the sclera, which functions in accommodation (i.e. the
ability of the eye to focus on near or distant objects by
changing the shape of the lens)
Iris – the smallest part of the vascular tunic, suspended
between the cornea and the lens

17. Cont…
– A flat ring of tissue attached at its periphery to the
sclera (by pectinate ligament) and to the ciliary body
– The iris divides the space between cornea and lens
into anterior and posterior chambers.
The opening at its center is the pupil through which light
enters the posterior part of the eye.
– The size of the pupil is regulated by smooth sphincter
and dilator muscles of the iris.
– The iris is the colored and pigmented part of the eye
– The number and type of pigments in the cells of the
stroma of iris determines the color of iris and the eye.
Eg. – if pigmented (melanin) cells are tightly packed –
iris is dark-brown
-if pigmented (melanin) cells are fewer – lighter and
yellowish

18. Cont…
3. Nervous tunic
• It is the internal tunic of the eye ball
• It contains the light-sensitive receptor cells,
Retina
Retina – is an extension of the brain by the optic
nerve
– It begins from the nerve that penetrates the choroids
at the optic disc and ends at the border of ciliary body
– Optic disc (blind spot) is an area where the nerve
fibers concentrate to leave the eye
– No receptor cell in optic disc, in contrast, an area of
maximum optical resolution (macula) is located a
short distance dorsolateral to optic disc

19. Cont…
– Only the posterior 2/3
rd of the retina (pars optica
retinae) can be reached by light entering through the
pupil. Only this part is provided with receptor cells. It
is relatively thick
– The remaining 1/3
rd
(pars ceca retinae) is “blind”,
mainly represented by thin pigmented layer that
continues to the ciliary body and back of iris.
– The presence of so much retinal and choroidal
pigment makes the interior of the posterior part of
eye dark (like the inside of the camera) so that the
pupil appears black.
– Retina has tow kinds of photoreceptor cells.
Rods – respond to dim light
Cones – color receptors

20. The refractive media of the eye
• It includes the structures that are transparent and
enables light to enter the eye, and play a role in
light rays refraction.
Cornea, Aqueous humor, Lens and Vitreous humor.
• Aqueous humor– is a clear watery fluid that fills
the space between cornea and lens.
– It is produced by cells of ciliary processes
– The interference with the drainage of aqueous humor
allows excess fluid to accumulate causing intraocular
pressure rise=>Glaucoma

22. Cont…
–In healthy eye=> Rate of production = Rate of
drainage=> maintaining constant pressure
–=> Rate of production > Rate of drainage=>
Glaucoma in man
Lens - is a solid biconvex structure sufficiently
elastic to be able to change in shape
–It has an outer capsule, which is thick at the
center of the lens where zonular fibers are
secured or attached
–The substance of the lens consists of very
regularly arranged fibers.
These fibers form concentric sheets that can be
peeled off like the layer of onion

23. Contd.
Vitreous humor- it is gel-like mass consisting mainly of
water and a stroma of fine transparent fibers
– It occupies the space between the lens and retina

24. Accessory Organs of the Eye (Accessory Ocular
Structures)
– They are structures that help to move and protect the
eye ball, and they are contained within orbit
– Accessory organs of the eye include eyelids,
conjunctiva, lacrimal apparatus, ocular muscles and
ocular fascia
The eye lids (plapebrae)
They are two musculofibrous folds (upper and lower)
The upper is more extensive and more mobile.
The point of junction between the upper and lower
eye lids (lateral and medial commissure or canthus)
bound the opening (palpebral tissue or fissure)

25. Cont…
 The eyelids consist of three layers –
The skin- outer layer
The musculo fibrous- middle layer
The mucosal- inner layer
 The skin is thin and delicate layer covered by
short hairs; on its free edges it has tactile hairs
(eye lashes or cilia).
 Musculofibrous layer is formed by the orbicularis
ocuil, orbital septum and the aponeurosis of
levator muscle.
 The Mucosal layer or inner surface of eyelid is
lined with a thin transparent mucous membrane
called palpebral conjunctiva

27. Contd.
The conjunctiva
Thin, transparent mucous membrane that lines
the inner surface of palpebrae and anterior
surface of eyeball (except the clear cornea)
There are two types of conjunctiva called
palpebral and bulbar conjunctiva
Palpebral conjunctiva- lines inner surface of
eye lids
Bulbar conjunctiva- lines the sclera and ends
at the limbus
The potential space between eyelids and the
eye ball is known as conjunctival sac

28. Cont…
The transparency of conjunctiva renders small
blood vessels to be visible
e.g. they are congested in infection and pale color of
conjunctiva suggests anemia, shock and internal
hemorrhage
A slight mucosal elevation in the medial angle of
eye is known as lacrimal caruncle.
Between the lacrimal caruncle and eye ball at
medial canthus of eye, there is a transverse
sheet of thin mucous membrance called third
eyelid or nictating membrance, which is lined
by mucous membrane and supported by T-
shaped ligament and contain glands.
Prolaple of 3rd eyelid in horse is typical sign of
Tetanus

29. 3rd eyelid

30. Cont…
Lacrimal apparatus
It consists of lacrimal gland proper, the glands in the
3rd eyelid and the duct system carrying lacrimal fluids.
The lacrimal gland is located between the eyeball and
dorsolateral wall of the orbit
It opens by ducts and drains its secretion into dorsal
extremity (fornix) of the conjunctival sac

31. Lacrimal Apparatus

32. Cont…
The secreted lacrimal fluid is pooled as lacrimal
lake (shallow depression surrounding lacrimal
caruncle)
=> the fluid pass to the duct system through Lacrimal
canaliculus => lacrimal sac => nasolacrimal duct =>
finally, opens and discharges into nasal cavity

34. Ocular muscles
 They are muscles attached to eyeball
 These include:
4 rectus muscles (inferior, superior, medial and
lateral)
2 Oblique muscles (inferior and superior) and
Retractor bulbi muscle

35. Cont…
• Except the inferior oblique muscle, all are
originated from the vicinity of the optic foramen at
the apex of the orbit.
• inferior oblique muscle originates from the
depression in the medioventral wall of the orbit.

36. Ocular muscle

37. The muscles are attached to the sclera of the
eye at one end and are anchored to the bony
orbit of the eye at their opposite ends.
Contraction of the muscles produce
movement of the eyes within the orbit

38. Contd.
Orbital fascia:- is a fascial layer
surrounding the eyeball within the orbit.
 The eyeball is surrounded by three conical
fascial layers (periorbital, superficial and
deep)

39. 2. Organs of Hearing and Equilibrium (Auditory System)
• The ears are also highly developed organs of
special senses and consists of three divisions:
I. External ear
II. Middle ear and
III. Inner ear
• The ear contains receptors which respond to
movement of head and convert sound waves
into nerve impulse.

40.  The auditory system is designed to detect and analyze sound in
the environment.
 Much of animal communication relies on this system.
 Hearing requires at least one ear;
 however, localization of sound requires two ears as the auditory
system must detect the difference in time of arrival or intensity of
sound impinging on the two ears.
 Hearing is a function of the cerebral cortex, whereas the auditory
reflex, such as turning the head in response to sound, is mediated
by the brainstem.
 Hearing involves the external ear, middle ear, and inner ear.

41. Frontal: (Aggression, smell and
mood)
Parietal: (reception and
evaluation of sensory info)
Temporal: (Smell, Hearing,
Memory and thought)
Occipital: (Visual processing)

42. The external ear directs sound waves into the ear
canal.
Sound waves induce vibration of the tympanic
membrane that separates the external ear from the
middle ear.
The middle ear is an air‐filled tympanic cavity,
separated from the external ear by the tympanic
membrane and from the inner ear by the vestibular
window and cochlear window.

43. Cont’d
 The middle ear contains a chain of three auditory ossicles;
malleus,
 incus, and
stapes
 Three ossicles (small bones) link the tympanic membrane with
the cochlea of the inner ear.
 Act as a system of levers that transmit sound wave
vibrations from the tympanic membrane to the cochlea
 Eustachian tube(auditory tube) connects the middle ear cavity
with the nasopharynx;
 equalize the pressure in the tympanic cavity with the pressure in
the external auditory canal.

45. Cont’d
• Malleus: outermost bone;
attached to tympanic
membrane
• Incus: middle bone
• Stapes: medial-most bone;
attached to membrane that
covers the oval window of
the cochlea

46. I. The External Ear
• It consists of two parts
 the auricle and
external acoustic meatus
Auricle (pinna):- it is part of the ear that sticks out
from the head
It is funnel shaped structure that opens wide distally
to receive sound and that rolls up to form a tube and
bends medially to continue with external acoustic
meatus
It is made up of elastic cartilage
External acoustic meatus- tubular extension of the
pinna and extends up to tympanic membrane
(Ear drum)
The canal has cartilaginous and osseous part

48. Cont…
It is lined by the skin that contains sebaceous and
ceruminous gland (produce ear wax (cerumen is
brown waxy substance that protects the canals)).
• The basis of external ear consists of:
Auricular cartilage– elastic cartilage that is somewhat
funnel shaped
Annular cartilage– adjacent to the base of auricular
cartilage and forms external acoustic canal
Scutiform cartilage– lies medial to auricular cartilage
and serves as a fulcrum for attached auricular
muscles.
• The auricular voluntary muscles are responsible
for movement of the ear towards the source of
sound

49. II. The middle ear
• It is housed in the temporal bone with the small
air filled space (cavity), tympanic cavity
• The cavity is lined by a thin mucous membrane.
• It communicates with the nasopharynx by
auditory (Eustachian) tube
• Separated from external ear by tympanic
membrane, and from inner ear by membrane
that closes the oval and round windows
• The tympanic cavity contains three auditory
ossicles, which mediate the transmission of
sound waves across the middle ear.

50. Cont…
 The names of ossicles in latero-medial sequence:
Malleus (hammer)
Incus (anvil) and
Stapes (stirrup)
 There are also two striated muscles in the middle
ear:
Tensor tympani
Stapedius
function in modulate the way sound
vibration are transmitted to chochlea

52. III. The inner ear
• It is the essential portion of organs of
hearing and equilibrium
• Mechanical stimuli produced by sound, and
positional changes of head are transformed
into nerve impulse at inner ear
• Because of the complexity of its shape =>
Labyrinth, it is divided into two parts:
1. Osseous labyrinth
2. Membranous labyrinth
1. Osseous labyrinth:- a series of cavities
within the petrous part of temporal bone

53. Cont…
• The cavity is divided into three portions:
a) Vestibule- central part of the osseous
labyrinth
It is oval in outline and communicates with
semicircular canal and cochlea
b) Semicircular canals- three curved bony tubes
that lie at right angle to each other
c) Cochlea- a conical and helical cavity that
resembles a snail shell
2. Membranous Labyrinth:- a series of closed
sacs and ducts contained within the
osseous labyrinth

54. Cont…
• The membranous labyrinth is divided into four
ducts:
Utricle
Saccule
Semicircular duct- in semicircular-canal
Cochlear duct- in cochlea
housed in osseous labyrinth

56. What is the sense of hearing and balance?
• The ear is a sensory organ that picks up
sound waves, allowing us to hear.
• It is also essential to our sense of balance:
– the organ of balance (the vestibular system) is
found inside the inner ear.
– It is made up of
• three semicircular canals
• utricle and
• the saccule.

57. For balance
• There are two sets of end organs in the inner
ear, or labyrinth: the semicircular canals,
which respond to rotational movements
(angular acceleration); and the utricle
(forward) and saccule (up and down) within
the vestibule, which respond to changes in
the position of the head with respect to
gravity (linear acceleration).

58. What are the 3 receptors responsible for balance?
• Maintaining balance depends on information
received by the brain from three peripheral
sources:
– eyes, muscles and joints, and vestibular organs
– All three of these information sources send signals
to the brain in the form of nerve impulses from
special nerve endings called sensory receptors.

59. What are the hearing receptor in the inner ear called?
• The cochlea is filled with two fluids
(endolymph and perilymph),
• inside the cochlea is the sensory receptor —
the Organ of Corti — which contains sensory
cells with hair-like structures (hair cells) that
are the nerve receptors for hearing.

60. What are sound receptor called?
• In the cochlea, the snail-shell-shaped
structure in the inner ear, about four
thousand specialized sound receptors
called inner hair cells convert vibrations into
electrical impulses that are sent to the brain.

61. 3. Organ of smell (Olfaction)
• Much better developed in domestic animals
than in human being
• The olfactory organ is situated in the nose
• It occupies a large area of olfactory mucosa
covering the lateral wall and the ethimoidal
conchae in the caudal part of nasal cavity
• The nerve impulse is carried to brain by
olfactory nerve

62. Cont’d..
• The olfactory system consists of the main
olfactory epithlium in the nasal cavity where
transduction of volatile odors occurs, and the
main olfactory bulb and its connections with
other parts of the brain

64. Cont’d
In general, Smell ocurs;
• Hair-like processes project
up from olfactory cells into
the mucous layer that
covers the nasal epithelium.
• Odor molecules dissolve in
the mucus and contact the
sensory processes
– Nerve impulses are
generated, travel to the
brain, and are
interpreted as particular
smells.

65. 4. Organ of taste (gustatory organ)
• The receptors for sense of taste are the
taste buds
Distributed over the tongue
• The taste buds are enclosed in the papillage
(gustatory papillage).
These include fungi form, vallate or foliate
papillae.
• The principal taste sensations are
sweetness, sourness and saltiness.

67. Cont’d
The taste buds for meaty tastes are
primarily located in the rostral two‐thirds
of the dorsal surface of the tongue.
The rostral and lateral portions of the
tongue are sensitive to sweet taste.
Salty and sour taste buds are most sensitive
on the lateral sides, but more caudal to the
area occupied by sweet taste buds.
However, the taste buds for salt occupy
only a small area.
The caudal portion of the tongue is most
sensitive to bitter tastes.
The taste receptor cells live for about 10
days and have to be replaced.

69. • Chemicals must dissolve in saliva, diffuse into taste
pores, and contact gustatory hairs before they can
be tasted. A food chemical is known as a tastant.
When a tastant binds to receptors in a gustatory
epithelial cell membrane, a graded depolarizing
potential is induced, causing neurotransmitter
release. Although taste cells in taste buds are
similar in appearance, there are following five
types:
– ■■ Those that sense sweetness (such as sugar)
– ■■ Those that sense sourness (such as lemons)
– ■■ Those that sense saltiness (such as table salt)
– ■■ Those that sense bitterness (such as caffeine)

70. Organs of general Senses
• the general senses are the senses which are
perceived due to receptors scattered throughout
the body such as touch, temperature, and hunger,
rather than tied to a specific structure, as the
special senses vision or hearing are

71. Organs of general Senses
1. Cutaneous Senses
• It includes the sensation of touch, pressure, pain,
heat and cold.
• any of the senses that are dependent on
receptors in the skin sensitive to contact,
pressure, vibration, temperature, or pain. Also
called skin sense
• The sensory receptors of the skin can be divided
into:
Free nerve endings:- nerve fibers that terminate either
as points or button like swellings in the epidermis.
They are pain receptors, heat and cold

72. Contd.
Lamellar – concentric lamellae in the subcutis
- Pressure receptor
Merkel disk receptor – Small cup-shaped discs at
the end of nerve fibers in dermis and adjacent
epidermis
-touch receptor

73. Thankyou
The end!!