# Anatomy and Physiology of Animals/The Senses

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## Objectives

After completing this section, you should know:

-   that the general senses of touch, pressure, pain etc. are situated
    in the dermis of the skin and in the body
-   that the special senses include those of smell, taste, sight,
    hearing, and balance
-   the main structures of the eye and their functions
-   the route taken by light through the eye to the retina
-   the role of the rods and cones in the retina
-   the advantages of binocular vision
-   the main structures of the ear and their functions
-   the route taken by sound waves through the ear to the cochlea
-   the role of the vestibular organ (semicircular canals and otolith
    organ) in maintaining balance and posture

## The sense organs

Sense organs allow animals to sense changes in the environment around
them and in their bodies so that they can respond appropriately. They
enable animals to avoid hostile environments, sense the presence of
predators and find food.

Animals can sense a wide range of stimuli that includes, touch,
pressure, pain, temperature, chemicals, light, sound, movement and
position of the body. Some animals can sense electric and magnetic
fields. All sense organs respond to stimuli by producing nerve impulses
that travel to the brain via a sensory nerve. The impulses are then
processed and interpreted in the brain as pain, sight, sound, taste etc.

The senses are often divided into two groups:

:   1\. The **general senses** of touch, pressure, pain and temperature
    that are distributed fairly evenly through the skin. Some are found
    in muscles and within joints.

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:   2\. The **special senses** which include the senses of smell, taste,
    sight, hearing and balance. The special sense organs may be quite
    complex in structure.

## Touch And Pressure

Within the dermis of the skin are numerous modified nerve endings that
are sensitive to touch and pressure. The roots of hairs may also be well
supplied with sensory receptors that inform the animal contact with an
object (see diagram 15.1). Whiskers are specially modified hairs.

## Pain

Receptors that sense pain are found in almost every tissue of the body.
They tell the animal that tissues are dangerously hot, cold, compressed
or stretched or that there is not enough blood flowing in them. The
animal may then be able to respond and protect itself from further
damage

![](Anatomy_and_physiology_of_animals_General_senses_in_skin.jpg "Anatomy_and_physiology_of_animals_General_senses_in_skin.jpg")

Diagram 15.1 - The general senses in the skin

## Temperature

Nerve endings in the skin respond to hot and cold stimuli (See diagram
15.1).

## Awareness Of Limb Position

There are sense organs in the muscles, tendons and joints that send
continuous impulses to the brain that tell it where each limb is. This
information allows the animal to place its limbs accurately and know
their exact position without having to watch them.

## Smell

Animals use the sense of smell to locate food, mark their territory,
identify their own offspring and the presence and sexual condition of a
potential mate. The organ of smell (**olfactory organ**) is located in
the nose and responds to chemicals in the air. It consists of modified
nerve cells that have several tiny hairs on the surface. These emerge
from the epithelium on the roof of the nose cavity into the mucus that
lines it. As the animal breathes, chemicals in the air dissolve in the
mucus. When the sense cell responds to a particular molecule, it fires
an impulse that travels along the **olfactory nerve** to the brain where
it is interpreted as an odour (see diagram 15.2).

![](Anatomy_and_physiology_of_animals_Olfactory_organ_the_sense_of_smell.jpg "Anatomy_and_physiology_of_animals_Olfactory_organ_the_sense_of_smell.jpg")

Diagram 15.2 - The olfactory organ - the sense of smell

The olfactory sense in humans is rudimentary compared to that of many
animals. Carnivores that hunt have a very highly developed sensitivity
to scents. For example a polar bear can smell out a dead seal 20 km away
and a bloodhound can distinguish between the trails of different people
although it may sometimes be confused by the criss-crossing trail of
identical twins.

Snakes and lizards detect odours by means of **Jacobson's organ**. This
is situated on the roof of the mouth and consists of pits containing
sensory cells. When snakes flick out their forked tongues they are
smelling the air by carrying the molecules in it to the Jacobson's
organ.

## Taste

The sense of taste allows animals to detect and identify dissolved
chemicals. In reptiles, birds, and mammals the taste receptors (**taste
buds**) are found mainly to the upper surface of the tongue. They
consist of pits containing sensory cells arranged rather like the
segments of an orange (see diagram 15.3). Each receptor cell has a tiny
"hair" that projects into the saliva to sense the chemicals dissolved in
it.

![](Anatomy_and_physiology_of_animals_Taste_buds_on_the_tongue.jpg "Anatomy_and_physiology_of_animals_Taste_buds_on_the_tongue.jpg")

Diagram 15.3. Taste buds on the tongue

The sense of taste is quite restricted. Humans can only distinguish four
different tastes (sweet, sour, bitter and salt) and what we normally
think of as taste is mainly the sense of smell. Food is quite tasteless
when the nose is blocked and cats often refuse to eat when this happens.

## Sight

The eyes are the organs of sight. They consist of spherical **eyeballs**
situated in deep depressions in the skull called the **orbits**. They
are attached to the wall of the orbit by six muscles, which move the
eyeball. Upper and lower **eyelids** cover the eyes during sleep and
protect them from foreign objects or too much light, and spread the
tears over their surface.

The **nictitating membrane** or **haw** is a transparent sheet that
moves sideways across the eye from the inner corner, cleansing and
moistening the cornea without shutting out the light. It is found in
birds, crocodiles, frogs and fish as well as marsupials like the
kangaroo. It is rare in mammals but can be seen in cats and dogs by
gently opening the eye when it is asleep. **Eyelashes** also protect the
eyes from the sun and foreign objects.

### Structure of the Eye

Lining the eyelids and covering the front of the eyeball is a thin
epithelium called the **conjunctiva**. Conjunctivitis is inflammation of
this membrane. **Tear glands** that open just under the top eyelid
secrete a salty solution that keeps the exposed part of the eye moist,
washes away dust and contains an enzyme that destroys bacteria.

The wall of the eyeball is composed of three layers (see diagram 15.4).
From the outside these are the **sclera**, the **choroid** and the
**retina**.

![](Anatomy_and_physiology_of_animals_Structure_of_the_eye.jpg "Anatomy_and_physiology_of_animals_Structure_of_the_eye.jpg")

Diagram 15.4 - The structure of the eye

The **sclera** is a tough fibrous layer that protects the eyeball and
gives it rigidity. At the front of the eye the sclera is visible as the
"**white**" of the eye, which is modified as the transparent **cornea**
through which the light rays have to pass to enter the eye. The cornea
helps focus light that enters the eye.

The **choroid** lies beneath the sclera. It contains a network of blood
vessels that supply the eye with oxygen and nutrients. Its inner surface
is highly pigmented and absorbs stray light rays. In nocturnal animals
like the cat and possum this highly pigmented layer reflects light as a
means of conserving light. This is what makes them shine when caught in
car headlights.

At the front of the eye the choroid becomes the **iris**. This is the
coloured part of the eye that controls the amount of light entering the
**pupil** of the eye. In dim light the pupil is wide open so as much
light as possible enters while in bright light the pupils contract to
protect the retina from damage by excess light.

The **pupil** in most animals is circular but in many nocturnal animals
it is a slit that can close completely. This helps protect the
extra-sensitive light sensing tissues of animals like the cat and possum
from bright sunlight.

The inner layer lining the inside of the eye is the **retina**. This
contains the light sensing cells called **rods** and **cones** (see
diagram 15.5).

The **rod cells** are long and fat and are sensitive to dim light but
cannot detect colour. They contain large amounts of a pigment that
changes when exposed to light. This pigment comes from vitamin A found
in carrots etc. A deficiency of this vitamin causes night blindness. So
your mother was right when she told you to eat your carrots as they
would help you see in the dark!

The **cone cells** provide colour vision and allow animals to see
details. Most are found in the centre of the retina and they are most
densely concentrated in a small area called the **fovea**. This is the
area of sharpest vision, where the words you are reading at this moment
are focussed on your retina.

![](Anatomy_and_physiology_of_animals_A_rod_and_cone_from_the_retina.jpg "Anatomy_and_physiology_of_animals_A_rod_and_cone_from_the_retina.jpg")

Diagram 15.5 - A rod and cone from the retina

The nerve fibres from the cells of the retina join and leave the eye via
the **optic nerve**. There are no rods or cones here and it is a **blind
spot**. The optic nerve passes through the back of the orbit and enters
the brain.

The **lens** is situated just behind the pupil and the iris. It is a
crystalline structure with no blood vessels and is held in position by a
ligament. This is attached to a muscle, which changes the shape of the
lens so both near and distant objects can be focussed by the eye. This
ability to change the focus of the lens is called **accommodation**. In
many mammals the muscles that bring about accommodation are poorly
developed, Rats, cows and dogs, for example, are thought to be unable to
focus clearly on near objects.

In old age and certain diseases the lens may become cloudy resulting in
blurred vision. This is called a **cataract**. Within the eyeball are
two cavities, the **anterior and posterior chambers**, separated by the
lens. They contain fluids the **aqueous and vitreous
humours**respectively, that maintain the shape of the eyeball and help
press the retina firmly against the choroid so clear images are seen.

### How The Eye Sees

Eyes work quite like a camera. Light rays from an object enter the eye
and are focused on the retina (the "film") at the back of the eye. The
cornea, the lens and the fluid within the eye all help to focus the
light. They do this by bending the light rays so that light from the
object falls on the retina. This bending of light is called
**refraction**. The light stimulates the light sensitive cells of the
retina and nerve impulses are produced that pass down the optic nerve to
the brain (see diagram 15.6).

![](Anatomy_and_physiology_of_animals_How_light_travels_from_the_object_to_the_retina_of_the_eye.jpg "Anatomy_and_physiology_of_animals_How_light_travels_from_the_object_to_the_retina_of_the_eye.jpg")

Diagram 15.6 - How the light travels from the object to the retina of
the eye

### Colour Vision In Animals

As mentioned before, the retina has _two_
different kinds of cells that are stimulated by light -
***\'rodsandcones***\'. In humans and higher primates like baboons and
gorillas the rods function in dim light and do not perceive colour,
while the cones are stimulated by bright light and perceive details and
colour.

Other mammals have very few cones in their retinas and it is believed
that they see no or only a limited range of colour. It is, of course,
difficult to find out exactly what animals do see. It is thought that
deer, rats, and rabbits and nocturnal animals like the cat are
colour-blind, and dogs probably see green and blue. Some fish and most
birds seem to have better colour vision than humans and they use colour,
often very vivid ones, for recognizing each other as well as for
courtship and protection.

### Binocular Vision

Animals like cats that hunt have eyes placed on the front of the head in
such a way that both eyes see the same wide area but from slightly
different angles (see diagram 15.7). This is called binocular**vision**.
Its main advantage is that it enables the animals to estimate the
distance to the prey so they can chase it and pounce accurately.

![](Anatomy_and_physiology_of_animals_Well_developed_binocular_vision.jpg "Anatomy_and_physiology_of_animals_Well_developed_binocular_vision.jpg")

Diagram 15.7 - Well developed binocular vision in predator animals like
the cat

In contrast plant-eating prey animals like the rabbit and deer need to
have a wide panoramic view so they can see predators approaching. They
therefore have eyes placed on the side of the head, each with its own
field of vision (see diagram 15.8). They have only a very small area of
binocular vision in front of the head but are extremely sensitive to
movement.

![](Anatomy_and_physiology_of_animals_Panaoramic_monocular_vision.jpg "Anatomy_and_physiology_of_animals_Panaoramic_monocular_vision.jpg")

Diagram 15.8 - Panoramic monocular vision in prey animals like the
rabbit

## Hearing

Animals use the sense of hearing for many different purposes. It is used
to sense danger and enemies, to detect prey, to identify prospective
mates and to communicate within social groups. Some animals (e.g. most
bats and dolphins) use sound to "see" by echolocation. By sending out a
cry and interpreting the echo, they sense obstacles or potential prey.

### Structure of the Ear

Most of the ear, the organ of hearing, is hidden from view within the
bony skull. It consists of three main regions: the **outer ear, the
middle ear** and the **inner ear** (see diagram 15.9).

![](Anatomy_and_physiology_of_animals_The_ear.jpg "Anatomy_and_physiology_of_animals_The_ear.jpg")

Diagram 15.9 - The ear

The **outer ear** consists of an **ear canal** leading inwards to a thin
membrane known as the **eardrum**or **tympanic membrane** that stretches
across the canal. Many animals have an external ear flap or **pinna** to
collect and funnel the sound into the ear canal. The pinnae (plural of
pinna) usually face forwards on the head but many animals can swivel
them towards the source of the sound.

In dogs the ear canal is long and bent and often traps wax or provides
an ideal habitat for mites, yeast and bacteria.

The **middle ear** consists of a cavity in the skull that is connected
to the **pharynx** (throat) by a long narrow tube called the
**Eustachian tube**. This links the middle ear to the outside air so
that the air pressure on both sides of the eardrum can be kept the same.
Everyone knows the uncomfortable feeling (and affected hearing) that
occurs when you drive down a steep hill and the unequal air pressures on
the two sides of the eardrum cause it to distort. The discomfort is
relieved when you swallow because the Eustachian tubes open and the
pressure on either side equalises.

Within the cavity of the middle ear are three of the smallest bones in
the body, the **auditory ossicles**. They are known as the hammer, the
anvil and the stirrup because of their resemblance to the shape of these
objects. These tiny bones articulate (move against) each other and
transfer the vibrations of the eardrum to the membrane covering the
opening to the inner ear.

The **inner ear** is a complicated series of fluid-filled tubes imbedded
in the bone of the skull. It consists of two main parts. These are the
**cochlea** where sound waves are converted to nerve impulses and the
**vestibular organ** that is associated with the sense of balance and
has no role in hearing (see later).

The **cochlea** looks rather like a coiled up snail shell. Within it
there are specialised cells with fine hairs on their surface that
respond to the movement of the fluid within the cochlea by producing
nervous impulses that travel to the brain along the **auditory nerve**.

### How The Ear Hears

Sound waves can be thought of as vibrations in the air. They are
collected by the ear pinna and pass down the ear canal where they cause
the eardrum to vibrate. (An interesting fact is that when you are
listening to someone speaking your eardrum vibrates at exactly the same
rate as the vocal cords of the person speaking to you).

The vibration of the eardrum sets the three tiny bones in the middle ear
moving against each other so that the vibration is transferred to the
membrane covering the opening to the inner ear. As well as transferring
the vibration, the tiny ear bones also amplify it. The three tiny bones
are called the stirrup, anvil, and hammer. They were called such of
their form. In the human ear this amplification is about 20 times while
in desert-dwelling animals like the kangaroo rat it is 100 times. This
acute hearing warns them of the approach of predators like owls and
snakes, even in the dark.

The vibration causes waves in the fluid in the inner ear that pass down
the cochlea. These waves stimulate the tiny hair cells to produce nerve
impulses that travel via the auditory nerve to the cerebral cortex of
the brain where they are interpreted as sound.

To summarise: The route sound waves take as they pass through the ear
is: **External ear \| tympanic membrane \| ear ossicles \| inner ear
\|cochlear \| hair cells**

The hair cells generate a nerve impulse that travels down the auditory
nerve to the brain.

Remember that sound waves do not pass along the Eustachian tube. Its
function is to equalise the air pressure on either side of the tympanic
membrane.

## Balance

The **vestibular organ** of the inner ear helps an animal maintain its
posture and keep balanced by monitoring the movement and position of the
head. It consists of two structures - the **semicircular canals** and
the **otolith organs**.

The **semicircular canals** (see diagram 15.10) respond to movement of
the body. They tell an animal whether it is moving up or down, left or
right. They consist of three canals set in three different planes at
right angles to each other so that movement in any direction can be
registered. The canals contain fluid and sense cells with fine hairs
that project into the fluid. When the head moves the fluid swirls in the
canals and stimulates the hair cells. These send nerve impulses along
the **vestibular nerve** to the **cerebellum**.

Note that the semicircular canals register acceleration and deceleration
as well as changes in direction but do not respond to movement that is
at a constant speed.

The **otolith organs** are sometimes known as gravity receptors. They
tell you if your head is tilted or if you are standing on your head.
They consist of bulges at the base of the semi circular canals that
contain hair cells that are covered by a mass of jelly containing tiny
pieces of chalk called **otoliths** (see diagram 15.10). When the head
is tilted, or moved suddenly, the otoliths pull on the hair cells, which
produce a nerve impulse. This travels down the **vestibular nerve** to
the **cerebellum**. By coordinating the nerve impulses from the
semicircular canals and otolith organs the cerebellum helps the animal
keep its balance.

![](Anatomy_and_physiology_of_animals_Olith_organs.jpg "Anatomy_and_physiology_of_animals_Olith_organs.jpg")

Diagram 15.10 - Otolith Organs

## Summary

-   Receptors for touch, pressure, pain and temperature are found in the
    skin. Receptors in the muscles, tendons and joints inform the brain
    of limb position.
-   The **olfactory organ** in the nose responds to chemicals in the air
    i.e. smell.
-   **Taste buds** on the tongue respond to a limited range of chemicals
    dissolved in saliva.
-   The eyes are the organs of sight. Spherical **eyeballs** situated in
    orbits in the skull have walls composed of 3 layers.
-   The tough outer **sclera** protects and holds the shape of the
    eyeball. At the front it becomes visible as the white of the eye and
    the transparent **cornea** that allows light to enter the eye.
-   The middle layer is the **choroid**. In most animals it absorbs
    stray light rays but in nocturnal animal it is reflective to
    conserve light. At the front of the eye it becomes the **iris** with
    muscles to control the size of the **pupil** and hence the amount of
    light entering the eye.
-   The inner layer is the **retina** containing the light receptor
    cells: the **rods** for black and white vision in dim light and the
    **cones** for colour and detailed vision. Nerve impulses generated
    by these cells leave the eye for the brain via the **optic nerve**.
-   The **lens** (with the cornea) helps focus the light rays on the
    retina. Muscles alter the shape of the lens to allow near and far
    objects to be focussed.
-   **Aqueous humour** fills the space immediately behind the cornea and
    keeps it in shape and **vitreous humour**, a transparent jelly-like
    substance, fills the space behind the lens allowing light rays to
    pass through to the retina.
-   The ear is the organ of hearing and balance.
-   The external **pinna** helps funnel sound waves into the ear and
    locate the direction of the sound. The sound waves travel down the
    external **ear canal** to the **eardrum** or **tympanic membrane**
    causing it to vibrate. This vibration is transferred to the
    **auditory ossicles** of the middle ear which themselves transfer it
    to the inner ear. Here receptors in the **cochlea** respond by
    generating nerve impulses that travel to the brain via the
    **auditory** (acoustic) nerve.
-   The **Eustachian tube** connects the middle ear with the pharynx to
    equalise air pressure on either side of the tympanic membrane.
-   The **vestibular organ** of the inner ear is concerned with
    maintaining balance and posture. It consists of the **semicircular
    canals** and the **otolith organs**.

## Worksheet

Senses Worksheet

## Test Yourself

1\. Where are the organs that sense pain, pressure and temperature
found?

2\. Which sense organ responds to chemicals in the air?

3\. Match the words in the list below with the following descriptions.

:   optic nerve \| choroid \| cornea \| aqueous humor \| retina \| cones
    \| iris \| vitreous humour \| sclera \| lens

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:   a\) Focuses light rays on the retina.
:   b\) Respond to colour and detail.
:   c\) Outer coat of the eyeball.
:   d\) Carries nerve impulses from the retina to the brain.
:   e\) The chamber behind the lens is filled with this.
:   f\) This layer of the eyeball reflects light in nocturnal animals
    like the cat.
:   g\) This is the transparent window at the front of the eye.
:   h\) This constricts in bright light to reduce the amount of light
    entering the eye.
:   i\) The light rays are focused on here by the lens and cornea.
:   j\) The chamber in front of the lens is filled with this.

4\. Add the following labels to the diagram of the ear below.

:   pinna \| Eustachian tube \| cochlea \| tympanic membrane \| external
    ear canal \| ear ossicles \| semicircular canals

![](Unlabeled_diagram_of_the_dog's_ear.jpg "Unlabeled_diagram_of_the_dog's_ear.jpg")

5\. What is the role of the Eustachian tube?

6\. What do the ear ossicles do?

7\. What is the role of the semicircular canals?

/Senses Test Yourself
Answers/

## Websites

-   <http://en.wikipedia.org/wiki/Sense> Wikipedia. The old faithful.
    You can explore here to your hearts desire. Try 'eye', 'ear',
    'taste' etc. but also 'equilibrioception', and 'echolocation'.
-   <http://www.bbc.co.uk/science/humanbody/body/factfiles/smell/smell_ani_f5.swf>
    BBC Science and Nature. BBC animation of (human) olfactory organ and
    smelling.
-   <http://www.bbc.co.uk/science/humanbody/body/factfiles/taste/taste_ani_f5.swf>
    BBC Science. BBC animation of (human) taste buds and tasting.
-   <http://web.archive.org/web/20071121213719/http://www.bishopstopford.com/faculties/science/arthur/Eye%20Drag%20%26%20Drop.swf>
    Eye Diagram. A diagram of the eye to label and test your knowledge.
-   <http://www.bbc.co.uk/science/humanbody/body/factfiles/hearing/hearing_animation.shtml>
    BBC on Hearing. BBC animation of hearing. Well worth looking at.
-   <http://www.wisc-online.com/objects/index_tj.asp?objid=AP1502> Ear
    Animation. Another great animation of the ear and hearing.
-   <http://www.bbc.co.uk/science/humanbody/body/factfiles/balance/balance_ani_f5.swf>
    BBC Balance Animation. An animation of the action of the otolith
    organ (called macula in this animation)

## Glossary

-   Link to
    Glossary

