# Anatomy and Physiology of Animals/The Senses

!original image by miss
pupik cc
by{width="400"}

## 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


# Anatomy and Physiology of Animals/Endocrine System

!original image by Denis
Gustavo cc
by{width="400"}

**PREPARED BY ARNOLD WAMUKOTA, BUSIA**

## Objectives

After completing this section, you should know:

-   The characteristics of endocrine glands and hormones
-   The position of the main endocrine glands in the body
-   The relationship between the pituitary gland and the hypothalamus
-   The main hormones produced by the two parts of the pituitary gland
    and their effects on the body
-   The main hormones produced by the pineal, thyroid, parathyroid and
    adrenal glands, the pancreas, ovary and testicle in regard to their
    effects on the body
-   What is meant by homeostasis and feedback control
-   The homeostatic mechanisms that allow an animal to control its body
    temperature, water balance, blood volume and acid/base balance

## The Endocrine System

In order to survive, animals must constantly adapt to changes in the
environment. The **nervous** and **endocrine systems** both work
together to bring about this adaptation. In general the nervous system
responds rapidly to short-term changes by sending electrical impulses
along nerves and the endocrine system brings about longer-term
adaptations by sending out chemical messengers called hormones into the
blood stream. In general Endocrine system is represented by a set of
heterogeneous structure and origin of formations capable of internal
secretion, ie the release of biologically active substances (hormones)
that flow directly into the bloodstream.

For example, think about what happens when a male and female cat meet
under your bedroom window at night. The initial response of both cats
may include spitting, fighting and spine tingling yowling - all brought
about by the nervous system. Fear and stress then activates the adrenal
glands to secrete the hormone **adrenaline** which increases the heart
and respiratory rates. If mating occurs, other hormones stimulate the
release of ova from the ovary of the female and a range of different
hormones maintains pregnancy, delivery of the kittens and lactation.

`                                                PREPARED  By ARNOLD WERANGAI`

**Evolution of endocrine systems**

The most primitive endocrine systems seem to be those of the
neurosecretory type, in which the nervous system either secretes
neurohormones (hormones that act on, or are secreted by, nervous tissue)
directly into the circulation or stores them in neurohemal organs
(neurons whose endings directly contact blood vessels, allowing
neurohormones to be secreted into the circulation), from which they are
released in large amounts as needed. True endocrine glands probably
evolved later in the evolutionary history of the animal kingdom as
separate, hormone-secreting structures. Some of the cells of these
endocrine glands are derived from nerve cells that migrated during the
process of evolution from the nervous system to various locations in the
body. These independent endocrine glands have been described only in
arthropods (where neurohormones are still the dominant type of endocrine
messenger) and in vertebrates (where they are best developed).

It has become obvious that many of the hormones previously ascribed only
to vertebrates are secreted by invertebrates as well (for example, the
pancreatic hormone insulin). Likewise, many invertebrate hormones have
been discovered in the tissues of vertebrates, including those of
humans. Some of these molecules are even synthesized and employed as
chemical regulators, similar to hormones in higher animals, by
unicellular animals and plants. Thus, the history of endocrinologic
regulators has ancient beginnings, and the major changes that took place
during evolution would seem to centre around the uses to which these
molecules were put.

**Vertebrate endocrine systems**

Vertebrates (phylum Vertebrata) are separable into at least seven
discrete classes that represent evolutionary groupings of related
animals with common features. The class Agnatha, or the jawless fishes,
is the most primitive group. Class Chondrichthyes and class Osteichthyes
are jawed fishes that had their origins, millions of years ago, with the
Agnatha. The Chondrichthyes are the cartilaginous fishes, such as sharks
and rays, while the Osteichthyes are the bony fishes. Familiar bony
fishes such as goldfish, trout, and bass are members of the most
advanced subgroup of bony fishes, the teleosts, which developed lungs
and first invaded land. From the teleosts evolved the class Amphibia,
which includes frogs and toads. The amphibians gave rise to the class
Reptilia, which became more adapted to land and diverged along several
evolutionary lines. Among the groups descending from the primitive
reptiles were turtles, dinosaurs, crocodilians (alligators, crocodiles),
snakes, and lizards. Birds (class Aves) and mammals (class Mammalia)
later evolved from separate groups of reptiles. Amphibians, reptiles,
birds, and mammals, collectively, are referred to as the tetrapod
(four-footed) vertebrates.

The human endocrine system is the product of millions of years of
evolution. and it should not be surprising that the endocrine glands and
associated hormones of the human endocrine system have their
counterparts in the endocrine systems of more primitive vertebrates. By
examining these animals it is possible to document the emergence of the
hypothalamic-pituitary-target organ axis, as well as many other
endocrine glands, during the evolution of fishes that preceded the
origin of terrestrial vertebrates.

**The hypothalamic-pituitary-target organ axis**

The hypothalamic-pituitary-target organ axes of all vertebrates are
similar. The hypothalamic neurosecretory system is poorly developed in
the most primitive of the living Agnatha vertebrates, the hagfishes, but
all of the basic rudiments are present in the closely related lampreys.
In most of the more advanced jawed fishes there are several
well-developed neurosecretory centres (nuclei) in the hypothalamus that
produce neurohormones. These centers become more clearly defined and
increase in the number of distinct nuclei as amphibians and reptiles are
examined, and they are as extensive in birds as they are in mammals.
Some of the same neurohormones that are found in humans have been
identified in nonmammals, and these neurohormones produce similar
effects on cells of the pituitary as described above for mammals.

Two or more neurohormonal peptides with chemical and biologic properties
similar to those of mammalian oxytocin and vasopressin are secreted by
the vertebrate hypothalamus (except in Agnatha fishes, which produce
only one). The oxytocin-like peptide is usually isotocin (most fishes)
or mesotocin (amphibians, reptiles, and birds). The second peptide is
arginine vasotocin, which is found in all nonmammalian vertebrates as
well as in fetal mammals. Chemically, vasotocin is a hybrid of oxytocin
and vasopressin, and it appears to have the biologic properties of both
oxytocin (which stimulates contraction of muscles of the reproductive
tract, thus playing a role in egg-laying or birth) and vasopressin (with
either diuretic or antidiuretic properties). The functions of the
oxytocin-like substances in non-mammals are unknown.

The pituitary glands of all vertebrates produce essentially the same
tropic hormones: thyrotropin (TSH), corticotropin (ACTH), melanotropin
(MSH), prolactin (PRL), growth hormone (GH), and one or two
gonadotropins (usually FSH-like and LH-like hormones). The production
and release of these tropic hormones are controlled by neurohormones
from the hypothalamus. The cells of teleost fishes, however, are
innervated directly. Thus, these fishes may rely on neurohormones as
well as neurotransmitters for stimulating or inhibiting the release of
tropic hormones.

Among the target organs that constitute the
hypothalamic-pituitary-target organ axis are the thyroid, the adrenal
glands, and the gonads. Their individual roles are discussed below.

**The thyroid axis**

Thyrotropin secreted by the pituitary stimulates the thyroid gland to
release thyroid hormones, which help to regulate development, growth,
metabolism, and reproduction. In humans, these thyroid hormones are
known as triiodothyronine (T3) and thyroxine (T4). The evolution of the
thyroid gland is traceable in the evolutionary development of
invertebrates to vertebrates. The thyroid gland evolved from an
iodide-trapping, glycoprotein-secreting gland of the protochordates (all
nonvertebrate members of the phylum Chordata). The ability of many
invertebrates to concentrate iodide, an important ingredient in thyroid
hormones, occurs generally over the surface of the body. In
protochordates, this capacity to bind iodide to a glycoprotein and
produce thyroid hormones became specialized in the endostyle, a gland
located in the pharyngeal region of the head. When these iodinated
proteins are swallowed and broken down by enzymes, the iodinated amino
acids known as thyroid hormones are released. Larvae of primitive
vertebrate lampreys also have an endostyle like that of the
protochordates. When a lamprey larva undergoes metamorphosis into an
adult lamprey, the endostyle breaks into fragments. The resulting clumps
of endostyle cells differentiate into the separate follicles of the
thyroid gland. Thyroid hormones actually direct metamorphosis in the
larvae of lampreys, bony fishes, and amphibians. Thyroids of fishes
consist of scattered follicles in the pharyngeal region. In tetrapods
and a few fishes, the thyroid becomes encapsulated by a layer of
connective tissue.

**The adrenal axis**

The adrenal axes in mammals and in nonmammals are not constructed along
the same lines. In mammals the adrenal cortex is a separate structure
that surrounds the internal adrenal medulla; the adrenal gland is
located atop the kidneys. Because the cells of the adrenal cortex and
adrenal medulla do not form separate structures in nonmammals as they do
in mammals, they are often referred to in different terms; the cells
that correspond to the adrenal cortex in mammals are called inter renal
cells, and the cells that correspond to the adrenal medulla are called
chromaffin cells. In primitive non mammals the adrenal glands are
sometimes called inter renal glands.

In fishes the interrenal and chromaffin cells often are embedded in the
kidneys, whereas in amphibians they are distributed diffusely along the
surface of the kidneys. Reptiles and birds have discrete adrenal glands,
but the anatomical relationship is such that often the "cortex" and the
"medulla" are not distinct units. Under the influence of pituitary
adrenocorticotropin hormone, the interrenal cells produce steroids
(usually corticosterone in tetrapods and cortisol in fishes) that
influence sodium balance, water balance, and metabolism.

**The gonadal axis**

Gonadotropins secreted by the pituitary are basically LH-like and/or
FSH-like in their actions on vertebrate gonads. In general, the FSH-like
hormones promote development of eggs and sperm and the LH-like hormones
cause ovulation and sperm release; both types of gonadotropins stimulate
the secretion of the steroid hormones (androgens, estrogens, and, in
some cases, progesterone) from the gonads. These steroids produce
effects similar to those described for humans. For example, progesterone
is essential for normal gestation in many fishes, amphibians, and
reptiles in which the young develop in the reproductive tract of the
mother and are delivered live. Androgens (sometimes testosterone, but
often other steroids are more important) and estrogens (usually
estradiol) influence male and female characteristics and behaviour.

**Control of pigmentation**

Melanotropin (melanocyte-stimulating hormone, or MSH) secreted by the
pituitary regulates the star-shaped cells that contain large amounts of
the dark pigment melanin (melanophores), especially in the skin of
amphibians as well as in some fishes and reptiles. Apparently, light
reflected from the surface stimulates photoreceptors, which send
information to the brain and in turn to the hypothalamus. Pituitary
melanotropin then causes the pigment in the melanophores to disperse and
the skin to darken, sometimes quite dramatically. By releasing more or
less melanotropin, an animal is able to adapt its colouring to its
background.

**Growth hormone and prolactin**

The functions of growth hormone and prolactin secreted by the pituitary
overlap considerably, although prolactin usually regulates water and
salt balance, whereas growth hormone primarily influences protein
metabolism and hence growth. Prolactin allows migratory fishes such as
salmon to adapt from salt water to fresh water. In amphibians, prolactin
has been described as a larval growth hormone, and it can also prevent
metamorphosis of the larva into the adult. The water-seeking behaviour
(so-called water drive) of adult amphibians often observed prior to
breeding in ponds is also controlled by prolactin. The production of a
protein-rich secretion by the skin of the discus fish (called "discus
milk") that is used to nourish young offspring is caused by a
prolactin-like hormone. Similarly, prolactin stimulates secretions from
the crop sac of pigeons ("pigeon" or "crop" milk), which are fed to
newly hatched young. This action is reminiscent of prolactin's actions
on the mammary gland of nursing mammals. Prolactin also appears to be
involved in the differentiation and function of many sex accessory
structures in nonmammals, and in the stimulation of the mammalian
prostate gland. For example, prolactin stimulates cloacal glands
responsible for special reproductive secretions. Prolactin also
influences external sexual characteristics such as nuptial pads (for
clasping the female) and the height of the tail in male salamanders.

**Other vertebrate endocrine glands**

**The pancreas**

The pancreas in nonmammals is an endocrine gland that secretes insulin,
glucagon, and somatostatin. Pancreatic polypeptide has been identified
in birds and may occur in other groups as well. Insulin lowers blood
sugar (hypoglycemia) in most vertebrates, although mammalian insulin is
rather ineffective in reptiles and birds. Glucagon is a hyperglycemic
hormone (it increases the level of sugar in the blood).

In primitive fishes the cells responsible for secreting the pancreatic
hormones are scattered within the wall of the intestine. There is a
trend toward progressive clumping of cells in more evolutionarily
advanced fishes, and in a few species the endocrine tissue forms only
one or a few large islets. As a rule, most fishes lack a discrete
pancreas, but all tetrapods have a fully formed exocrine and endocrine
pancreas. The endocrine cells of all tetrapods are organized into
distinct islets as described for humans, although the abundance of the
different cell types often varies. For example, in reptiles and birds
there is a predominance of glucagon-secreting cells and relatively few
insulin-secreting cells.

**Calcium-regulating hormones**

Fishes have no parathyroid glands: these glands first appear in
amphibians. Although the embryological origin of parathyroid glands of
tetrapods is well known, their evolutionary origin is not. Parathyroid
hormone raises blood calcium levels (hypercalcemia) in tetrapods. The
absence in most fishes of cellular bone, which is the principal target
for parathyroid hormone in tetrapods, is reflected by the absence of
parathyroid glands.

Fishes, amphibians, reptiles, and birds have paired pharyngeal
ultimobranchial glands that secrete the hypocalcemic hormone calcitonin.
The corpuscles of Stannius, unique glandular islets found only in the
kidneys of bony fishes, secrete a peptide called hypocalcin. Fish
calcitonins differ somewhat from the mammalian peptide hormone of the
same name, and fish calcitonins have proved to be more potent and have a
longer-lasting action in humans than human calcitonin itself.
Consequently, synthetic fish calcitonin has been used to treat humans
suffering from various disorders of bone, including Paget's disease. The
secretory cells of the ultimobranchial glands are derived from cells
that migrated from the embryonic nervous system. During the development
of a mammalian fetus, the ultimobranchial gland becomes incorporated
into the developing thyroid gland as the "C cells" or "parafollicular
cells."

**Gastrointestinal hormones**

Little research has been done on gastrointestinal hormones in
nonmammals, but there is good evidence for a gastrinlike mechanism that
controls the secretion of stomach acids. Peptides similar to
cholecystokinin are also present and can stimulate contractions of the
gall bladder. The gall bladders of primitive fishes contract when
treated with mammalian cholecystokinin.

**Other mammalian-like endocrine systems**

**The renin-angiotensin system**

The renin-angiotensin system in mammals is represented in nonmammals by
the juxtaglomerular cells that secrete renin associated with the kidney.
The macula densa that functions as a detector of sodium levels within
the kidney tubules of tetrapods, however, has not been found in fishes.

**The pineal complex**

In fishes, amphibians, and reptiles, the pineal complex is better
developed than in mammals. The nonmammalian pineal functions as both a
photoreceptor organ and an endocrine source for melatonin. Effects of
light on reproduction in fishes and tetrapods are mediated at least in
part through the pineal, and it has been implicated in a number of daily
and seasonal biorhythmic phenomena.

**Prostaglandins**

Many tissues of nonmammals produce prostaglandins that play important
roles in reproduction similar to those discussed for humans and other
mammals.

**The liver**

As in mammals, the liver of several nonmammalian species has been shown
to produce somatomedin-like growth factors in response to stimulation by
growth hormone. Similarly, there is evidence that prolactin stimulates
the production of a related growth factor, which synergizes (cooperates)
with prolactin on targets such as the pigeon crop sac.

**Unique endocrine glands in fishes**

In addition to the corpuscles of Stannius and the ultimobranchial
glands, most fishes have a unique neurosecretory neurohemal organ, the
urophysis, which is associated with the spinal cord at the base of the
tail. Although the functions of this caudal (rear) neurosecretory system
are not now understood, it is known to produce two peptides, urotensin I
and urotensin II. Urotensin I is chemically related to a family of
peptides that includes somatostatin; urotensin II is a member of the
family of peptides that includes mammalian corticotropin-releasing
hormone (CRH). There are no homologous structures to either the
corpuscles of Stannius or the urophysis in amphibians, reptiles, or
birds.

**Invertebrate endocrine systems**

Advances in the study of invertebrate endocrine systems have lagged
behind those in vertebrate endocrinology, largely due to the problems
associated with adapting investigative techniques that are appropriate
for large vertebrate animals to small invertebrates. It also is
difficult to maintain and study appropriately some invertebrates under
laboratory conditions. Nevertheless, knowledge about these systems is
accumulating rapidly.

All phyla in the animal kingdom that have a nervous system also possess
neurosecretory neurons. The results of studies on the distribution of
neurosecretory neurons and ordinary epithelial endocrine cells imply
that the neurohormones were the first hormonal regulators in animals.
Neurohemal organs appear first in the more advanced invertebrates (such
as mollusks and annelid worms), and endocrine epithelial glands occur
only in the most advanced phyla (primarily Arthropoda and Chordata).
Similarly, the peptide and steroid hormones found in vertebrates are
also present in the nervous and endocrine systems of many invertebrate
phyla. These hormones may perform similar functions in diverse animal
groups. With more emphasis being placed on research in invertebrate
systems, new neuropeptides are being discovered initially in these
animals, and subsequently in vertebrates.

The endocrine systems of some animal phyla have been studied in detail,
but the endocrine systems of only a few species are well known. The
following discussion summarizes the endocrine systems of five
invertebrate phyla and the two invertebrate subphyla of the phylum
Chordata, a phylum that also includes Vertebrata, a subphylum to which
the backboned animals belong

## Endocrine Glands And Hormones

Hormones are chemicals that are secreted by **endocrine glands**. Unlike
exocrine glands (see chapter 5), endocrine glands have no ducts, but
release their secretions directly into the blood system, which carries
them throughout the body. However, hormones only affect the specific
**target organs** that recognize them. For example, although it is
carried to virtually every cell in the body, **follicle stimulating
hormone** (FSH), released from the **anterior pituitary gland**, only
acts on the follicle cells of the ovaries causing them to develop.

A nerve impulse travels rapidly and produces an almost instantaneous
response but one that lasts only briefly. In contrast, hormones act more
slowly and their effects may be long lasting. Target cells respond to
minute quantities of hormones and the concentration in the blood is
always extremely low. However, target cells are sensitive to subtle
changes in hormone concentration and the endocrine system regulates
processes by changing the rate of hormone secretion.

The main endocrine glands in the body are the **pituitary, pineal,
thyroid, parathyroid**, and **adrenal glands**, the **pancreas,
ovaries** and **testes**. Their positions in the body are shown in
diagram 16.1.

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

Diagram 16.1 - The main endocrine organs of the body

## The Pituitary Gland And Hypothalamus

The **pituitary gland** is a pea-sized structure that is attached by a
stalk to the underside of the cerebrum of the brain (see diagram 16.2).
It is often called the "master" endocrine gland because it controls many
of the other endocrine glands in the body. However, we now know that the
pituitary gland is itself controlled by the **hypothalamus**. This small
but vital region of the brain lies just above the pituitary and provides
the link between the nervous and endocrine systems. It controls the
**autonomic nervous system**, produces a range of hormones and regulates
the secretion of many others from the pituitary gland (see Chapter 7 for
more information on the hypothalamus).

The pituitary gland is divided into two parts with different functions -
the **anterior** and **posterior pituitary** (see diagram 16.3).

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

Diagram 16.2 - The position of the pituitary gland and hypothalamus

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

Diagram 16.3 - The anterior and posterior pituitary

The **anterior pituitary gland** secretes hormones that regulate a wide
range of activities in the body. These include:

:   1\. **Growth hormone** that stimulates body growth.
:   2\. **Prolactin** that initiates milk production.
:   3\. **Follicle stimulating hormone (FSH**) that stimulates the
    development of the **follicles** of the ovaries. These then secrete
    **oestrogen** (see chapter 6).
:   4\. **melanocyte stimulating hormone (MSH**) that causes darkening
    of skin by producing melanin
:   5\. **lutenizing hormone (LH**) that stimulates ovulation and
    production of progesterone and testosterone

The **posterior pituitary gland**

:   1\. Antidiuretic Hormone (ADH), regulates water loss and increases
    blood pressure
:   2\. Oxytocin, milk \"let down\"

## The Pineal Gland

The **pineal gland** is found deep within the brain (see diagram 16.4).
It is sometimes known as the 'third eye" as it responds to light and day
length. It produces the hormone **melatonin**, which influences the
development of sexual maturity and the seasonality of breeding and
hibernation. **Bright light inhibits melatonin secretion** Low level of
melatonin in bright light makes one feel good and this increases
fertility. High level of melatonin in dim light makes an animal tired
and depressed and therefore causes low fertility in animals.

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

Diagram 16.4 - The pineal gland

## The Thyroid Gland

The **thyroid gland** is situated in the neck, just in front of the
windpipe or trachea (see diagram 16.5). It produces the hormone
**thyroxine**, which influences the rate of growth and development of
young animals. In mature animals it increases the rate of chemical
reactions in the body.

Thyroxine consists of 60% **iodine** and too little in the diet can
cause **goitre**, an enlargement of the thyroid gland. Many inland soils
in New Zealand contain almost no iodine so goitre can be common in stock
when iodine supplements are not given. To add to the problem, chemicals
called **goitrogens** that occur naturally in plants like kale that
belong to the **cabbage family**, can also cause goitre even when there
is adequate iodine available.

![](Anatomy_and_physiology_of_animals_Thyroid_&_parathyroid_glands.jpg "Anatomy_and_physiology_of_animals_Thyroid_&_parathyroid_glands.jpg")

Diagram 16.5 - The thyroid and parathyroid glands

## The Parathyroid Glands

The **parathyroid glands** are also found in the neck just behind the
thyroid glands (see diagram 16.5). They produce the hormone
**parathormone** that regulates the amount of **calcium** in the blood
and influences the excretion of **phosphates** in the urine.

## The Adrenal Gland

The **adrenal glands** are situated on the cranial surface of the
kidneys (see diagram 16.6). There are two parts to this endocrine gland,
an outer **cortex** and an inner **medulla**.

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

Diagram 16.6 - The adrenal glands

The **adrenal cortex** produces several hormones. These include:

:   1\. **Aldosterone** that regulates the concentration of **sodium and
    potassium** in the blood by controlling the amounts that are
    secreted or reabsorbed in the kidney tubules.
:   2\. **Cortisone** and **hydrocortisone** (cortisol) that have
    complex effects on glucose, protein and fat metabolism. In general
    they increase metabolism. They are also often administered to
    animals to counteract allergies and for treating arthritic and
    rheumatic conditions. However, prolonged use should be avoided if
    possible as they can increase weight and reduce the ability to heal.
:   3\. **Male and female sex hormones** similar to those secreted by
    the ovaries and testes.

The hormones secreted by the adrenal cortex also play a part in
"**general adaptation syndrome**" which occurs in situations of
prolonged stress.

The **adrenal medulla** secretes **adrenalin** (also called
**epinephrine**). Adrenalin is responsible for the so-called flight
fight, fright response that prepares the animal for emergencies. Faced
with a perilous situation the animal needs to either fight or make a
rapid escape. To do either requires instant energy, particularly in the
skeletal muscles. Adrenaline increases the amount of blood reaching them
by causing their blood vessels to dilate and the heart to beat faster.
An increased rate of breathing increases the amount of oxygen in the
blood and glucose is released from the liver to provide the fuel for
energy production. Sweating increases to keep the muscles cool and the
pupils of the eye dilate so the animal has a wide field of view.
Functions like digestion and urine production that are not critical to
immediate survival slow down as blood vessels to these parts constrict.

Note that the effects of adrenalin are similar to those of the
sympathetic nervous system.

`                                                              PREPARED BY ARNOLD WAMUKOTA`

## The Pancreas

In most animals the **pancreas** is an oblong, pinkish organ that lies
in the first bend of the small intestine (see diagram 16.7). In rodents
and rabbits, however, it is spread thinly through the mesentery and is
sometimes difficult to see.

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

Diagram 16.7 - The pancreas

Most of the pancreas acts as an **exocrine gland** producing digestive
enzymes that are secreted into the small intestine. The endocrine part
of the organ consists of small clusters of cells (called **Islets of
Langerhans**) that secrete the hormone **insulin**. This hormone
regulates the amount of **glucose** in the blood by increasing the rate
at which glucose is converted to glycogen in the liver and the movement
of glucose from the blood into cells.

In **diabetes mellitus** the pancreas produces insufficient insulin and
glucose levels in the blood can increase to a dangerous level. A major
symptom of this condition is glucose in the urine.

## The Ovaries

A part of the reproductive system of all female vertebrates. Although
not vital to individual survival, the ovary is vital to perpetuation of
the species. The function of the ovary is to produce the female germ
cells or ova, and in some species to elaborate hormones that assist in
regulating the reproductive cycle.

The ovaries develop as bilateral structures in all vertebrates, but
adult asymmetry is found in certain species of all vertebrates from the
elasmobranchs to the mammals.

The ovary of all vertebrates functions in essentially the same manner.
However, ovarian histology of the various groups differs considerably.
Even such a fundamental element as the ovum exhibits differences in
various groups. See Ovum

The mammalian ovary is attached to the dorsal body wall. The free
surface of the ovary is covered by a modified peritoneum called the
germinal epithelium. Just beneath the germinal epithelium is a layer of
fibrous connective tissue. Most of the rest of the ovary is made up of a
more cellular and more loosely arranged connective tissue (stroma) in
which are embedded the germinal, endocrine, vascular, and nervous
elements.

The most obvious ovarian structures are the follicles and the corpora
lutea. The smallest, or primary, follicle consists of an oocyte
surrounded by a layer of follicle (nurse) cells. Follicular growth
results from an increase in oocyte size, multiplication of the follicle
cells, and differentiation of the perifollicular stroma to form a
fibrocellular envelope called the theca interna. Finally, a fluid-filled
antrum develops in the granulosa layer, resulting in a vesicular
follicle.

The cells of the theca intima hypertrophy during follicular growth and
many capillaries invade the layer, thus forming the endocrine element
that is thought to secrete estrogen. The other known endocrine structure
is the corpus luteum, which is primarily the product of hypertrophy of
the granulosa cells remaining after the follicular wall ruptures to
release the ovum. Ingrowths of connective tissue from the theca interna
deliver capillaries to vascularize the hypertrophied follicle cells of
this new corpus luteum; progesterone is secreted here.

`                                                           PREPARED BY ARNOLD WAMUKOTA`

## The Testes

Sperm need temperatures between 2 and 10 degrees Centigrade lower and
then the body temperature to develop. This is the reason why the testes
are located in a bag of skin called the scrotal sacs (or scrotum) that
hangs below the body and where the evaporation of secretions from
special glands can further reduce the temperature. In many animals
(including humans) the testes descend into the scrotal sacs at birth but
in some animals they do not descend until sexual maturity and in others
they only descend temporarily during the breeding season. A mature
animal in which one or both testes have not descended is called a
cryptorchid and is usually infertile.

The problem of keeping sperm at a low enough temperature is even greater
in birds that have a higher body temperature than mammals. For this
reason bird's sperm are usually produced at night when the body
temperature is lower and the sperm themselves are more resistant to
heat.

The testes consist of a mass of coiled tubes (the seminiferous or sperm
producing tubules) in which the sperm are formed by meiosis (see diagram
13.4). Cells lying between the seminiferous tubules produce the male sex
hormone testosterone.

When the sperm are mature they accumulate in the collecting ducts and
then pass to the epididymis before moving to the sperm duct or vas
deferens. The two sperm ducts join the urethra just below the bladder,
which passes through the penis and transports both sperm and urine.

Ejaculation discharges the semen from the erect penis. It is brought
about by the contraction of the epididymis, vas deferens, prostate gland
and urethra.

`                                             PREPARED BY ARNOLD WAMUKOTA`

## Summary

-   **Hormones** are chemicals that are released into the blood by
    **endocrine glands** i.e. Glands with no ducts. Hormones act on
    specific **target organs** that recognize them.
-   The main endocrine glands in the body are the **hypothalamus,
    pituitary, pineal, thyroid, parathyroid** and **adrenal glands,**
    the **pancreas, ovaries** and **testes**.
-   The **hypothalamus** is situated under the **cerebrum** of the
    brain. It produces or controls many of the hormones released by the
    pituitary gland lying adjacent to it.
-   The **pituitary gland** is divided into two parts: the **anterior
    pituitary** and the **posterior pituitary**.
-   The **anterior pituitary** produces:

:\* **Growth hormone** that stimulates body growth

:\* **Prolactin** that initiates milk production

:\* **Follicle stimulating hormone** (**FSH**) that stimulates the
development of **ova**

:\* **Luteinising hormone (LH**) that stimulates the development of the
**corpus luteum**

:\* Plus several other hormones

-   The **posterior pituitary** releases:

:\* **Antidiuretic hormone** (ADH) that regulates **water loss** and
raises **blood pressure**

:\* **Oxytocin** that stimulates milk "let down".

-   The **pineal gland** in the brain produces **melatonin** that
    influences **sexual development** and **breeding cycles**.
-   The **thyroid gland** located in the neck, produces thyroxine, which
    influences the **rate of growth** and **development** of young
    animals. Thyroxine consists of 60% **iodine**. Lack of iodine leads
    to **goitre**.
-   The **parathyroid glands** situated adjacent to the thyroid glands
    in the neck produce **parathormone** that regulates blood
    **calcium** levels and the excretion of **phosphates**.
-   The **adrenal gland** located adjacent to the kidneys is divided
    into the outer **cortex** and the inner **medulla**.
-   The **adrenal cortex** produces:

:\***Aldosterone** that regulates the blood concentration of **sodium
and potassium**

:\* **Cortisone** and **hydro-cortisone** that affect **glucose,
protein** and **fat** metabolism

:\* Male and female **sex hormones**

-   The **adrenal medulla** produces **adrenalin** responsible for the
    **flight, fright, fight** response that prepares animals for
    emergencies.
-   The **pancreas** that lies in the first bend of the small intestine
    produces**insulin** that regulates blood **glucose** levels.
-   The **ovaries** are located in the lower abdomen produce 2 important
    sex hormones:

:\* The **follicle cells** of the developing ova produce **estrogen**,
which controls the development of the **mammary glands** and prepares
the uterus for pregnancy.

:\* The **corpus luteum** that develops in the empty **follicle** after
ovulation produces **progesterone**. This hormone further prepares the
**uterus** for pregnancy and maintains the pregnancy.

-   The **testes** produce **testosterone** that stimulates the
    development of the **male reproductive system** and **sexual
    characteristics**.

## Homeostasis and Feedback Control

Animals can only survive if the environment within their bodies and
their cells is kept constant and independent of the changing conditions
in the external environment. As mentioned in module 1.6, the process by
which this stability is maintained is called homeostasis. The body
achieves this stability by constantly monitoring the internal conditions
and if they deviate from the norm initiating processes that bring them
back to it. This mechanism is called feedback control. For example, to
maintain a constant body temperature the hypothalamus monitors the blood
temperature and initiates processes that increase or decrease heat
production by the body and loss from the skin so the optimum temperature
is always maintained. The processes involved in the control of body
temperature, water balance, blood loss and acid/base balance are
summarized below.

## Summary of Homeostatic Mechanisms

### 1. Temperature control

The biochemical and physiological processes in the cell are sensitive to
temperature. The optimum body temperature is about 37 C \[99 F\] for
mammals, and about 40 C \[104 F\] for birds. Biochemical processes in
the cells, particularly in muscles and the liver, produce heat. The heat
is distributed through the body by the blood and is lost mainly through
the skin surface. The production of this heat and its loss through the
skin is controlled by the hypothalamus in the brain which acts rather
like a thermostat on an electric heater. .

\(a\) When the body temperature rises above the optimum, a decrease in
temperature is achieved by:

-   Sweating and panting to increase heat loss by evaporation.

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-   Expansion of the blood vessels near the skin surface so heat is lost
    to the air.

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-   Reducing muscle exertion to the minimum.

\(b\) When the body temperature falls below the optimum, an increase in
temperature can be achieved by:

-   Moving to a heat source e.g. in the sun, out of the wind.

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-   Increasing muscular activity

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-   Shivering

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```
-   Making the hair stand on end by contraction of the hair erector
    muscles or fluffing of the feathers so there is an insulating layer
    of air around the body

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```
-   Constricting the blood vessels near the skin surface so heat loss to
    the air is decreased

### 2. Water balance

The concentration of the body fluids remains relatively constant
irrespective of the diet or the quantity of water taken into the body by
the animal. Water is lost from the body by many routes (see module 1.6)
but the kidney is the main organ that influences the quantity that is
lost. Again it is the hypothalamus that monitors the concentration of
the blood and initiates the release of hormones from the posterior
pituitary gland. These act on the kidney tubules to influence the amount
of water (and sodium ions) absorbed from the fluid flowing along them.

\(a\) When the body fluids become too concentrated and the osmotic
pressure too high, water retention in the kidney tubules can be achieved
by:

-   An increased production of anti-diuretic hormone (ADH) from the
    posterior pituitary gland, which causes more water to be reabsorbed
    from the kidney tubules.

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-   A decreased blood pressure in the glomerulus of the kidney results
    in less fluid filtering through into the kidney tubules so less
    urine is produced.

\(b\) When the body fluids become too dilute and the osmotic pressure
too low, water loss in the urine can be achieved by:

-   A decrease in the secretion of ADH, so less water is reabsorbed from
    the kidney tubules and more diluted urine is produced.

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```
-   An increase in the blood pressure in the glomerulus so more fluid
    filters into the kidney tubule and more urine is produced.

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```
-   An increase in sweating or panting that also increases the amount of
    water lost.

Another hormone, aldosterone, secreted by the cortex of the adrenal
gland, also affects water balance indirectly. It does this by increasing
the absorption of sodium ions (Na-) from the kidney tubules. This
increases water retention since it increases the osmotic pressure of the
fluids around the tubules and water therefore flows out of them by
osmosis.

### 3. Maintenance of blood volume after moderate blood loss

Loss of blood or body fluids leads to decreased blood volume and hence
decreased blood pressure. The result is that the blood system fails to
deliver enough oxygen and nutrients to the cells, which stop functioning
properly and may die. Cells of the brain are particularly vulnerable.
This condition is known as shock.

If blood loss is not extreme, various mechanisms come into play to
compensate and ensure permanent tissue damage does not occur. These
mechanisms include:

-   Increased thirst and drinking increases blood volume.

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-   Blood vessels in the skin and kidneys constrict to reduce the total
    volume of the blood system and hence retain blood pressure.

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```
-   Heart rate increases. This also increases blood pressure.

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```
-   Antidiuretic hormone (ADH) is released by the posterior pituitary
    gland. This increases water re-absorption in the collecting ducts of
    the kidney tubules so concentrated urine is produced and water loss
    is reduced. This helps maintain blood volume.

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```
-   Loss of fluid causes an increase in osmotic pressure of the blood.
    Proteins, mainly albumin, released into the blood by the liver
    further increase the osmotic pressure causing fluid from the tissues
    to be drawn into the blood by osmosis. This increases blood volume.

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```
-   Aldosterone, secreted by the adrenal cortex, increases the
    absorption of sodium ions (Na+) and water from the kidney tubules.
    This increases urine concentration and helps retain blood volume.

If blood or fluid loss is extreme and the blood volume falls by more
than 15-25%, the above mechanisms are unable to compensate and the
condition of the animal progressively deteriorates. The animal will die
unless a vet administers fluid or blood.

### 4. Acid/ base balance

Biochemical reactions within the body are very sensitive to even small
changes in acidity or alkalinity (i.e. pH) and any departure from the
narrow limits disrupts the functioning of the cells. It is therefore
important that the blood contains balanced quantities of acids and
bases.

The normal pH of blood is in the range 7.35 to 7.45 and there are a
number of mechanisms that operate to maintain the pH in this range.
Breathing is one of these mechanisms.

Much of the carbon dioxide produced by respiration in cells is carried
in the blood as carbonic acid. As the amount of carbon dioxide in the
blood increases the blood becomes more acidic and the pH decreases. This
is called acidosis and when severe can cause coma and death. On the
other hand, alkalosis (blood that is too alkaline) causes over
stimulation of the nervous system and when severe can lead to
convulsions and death.

\(a\) When vigorous activity generating large quantities of carbon
dioxide causes the blood to becomes too acidic it can be counteracted in
two ways:

-   By the rapid removal of carbon dioxide from the blood by deep,
    panting breaths

`   By the secretion of hydrogen ions (H+) into the urine by the kidney tubules. `

\(b\) When over breathing or hyperventilation results in low levels of
carbon dioxide in the blood and the blood is too alkaline, various
mechanisms come into play to bring the pH back to within the normal
range. These include:

-   A slower rate of breathing
-   A reduction in the amount of hydrogen ions (H+) secreted into the
    urine.

### SUMMARY

Homeostasis is the maintenance of constant conditions within a cell or
animal's body despite changes in the external environment.

The body temperature of mammals and birds is maintained at an optimum
level by a variety of heat regulation mechanisms. These include:

-   Seeking out warm areas,
-   Adjusting activity levels,

blood vessels on the body surface,

-   Contraction of the erector muscles so hairs and feathers stand up to
    form an insulating layer,
-   Shivering,
-   Sweating and panting in dogs.

Animals maintain water balance by:

-   adjusting level of antidiuretic hormone(ADH)
-   adjusting level of aldosterone,
-   adjusting blood flow to the kidneys
-   adjusting the amount of water lost through sweating or panting.

Animals maintain blood volume after moderate blood loss by:

-   Drinking,
-   Constriction of blood vessels in the skin and kidneys,
-   increasing heart rate,
-   secretion of anti-diuretic hormone
-   secretion of aldosterone
-   drawing fluid from the tissues into the blood by increasing the
    osmotic pressure of the blood.

Animals maintain the acid/base balance or pH of the blood by:

-   Adjusting the rate of breathing and hence the amount of CO2 removed
    from the blood.
-   Adjusting the secretion of hydrogen ions into the urine.

## Worksheet

Endocrine System
Worksheet

## Test Yourself

1\. What is Homeostasis?

2\. Give 2 examples of homeostasis

3\. List 3 ways in which animals keep their body temperature constant
when the weather is hot

4\. How does the kidney compensate when an animal is deprived of water
to drink

5\. After moderate blood loss, several mechanisms come into play to
increase blood pressure and make up blood volume. 3 of these mechanisms
are:

6\. Describe how panting helps to reduce the acidity of the blood

/Test Yourself Answers/

## Websites

-   <http://www.zerobio.com/drag_oa/endo.htm> A drag and drop hormone
    and endocrine organ matching exercise.

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```
-   <http://en.wikipedia.org/wiki/Endocrine_system> Wikipedia. Much,
    much more than you ever need to know about hormones and the
    endocrine system but with a bit of discipline you can glean lots of
    useful information from this site.

## Glossary

-   Link to
    Glossary


# Anatomy and Physiology of Animals/The Author

![](Ruth_-_small.jpg "Ruth_-_small.jpg"){width="300"}

Ruth Lawson is a zoologist who gained her first degree at Imperial
College, London University and her D.Phil from York University, UK.
After post graduate research on the tropical parasitic worm that causes
schistosomiasis, she emigrated to New Zealand where she spent 10 years
studying how hydatid disease spreads and can be controlled. With the
birth of her daughter, Kate, she started to teach at the Otago
Polytechnic, in Dunedin. Although human and animal anatomy and
physiology has been her main teaching focus, she retains a strong
interest and teaches courses in parasitology, public health, animal
nutrition and pig husbandry. Ruth lives on the Otago Peninsula
overlooking the beautiful Otago Harbour where she races her Topper
sailing dinghy. She also enjoys tramping, skiing and gardening and has
meditated for many years.


# Anatomy and Physiology of Animals/Acknowledgements

Many thanks to Terry Marler (B.V.S.C.) for his guiding vision, wisdom,
experience and patience. His advice throughout the writing of this
WikiBook has been invaluable. I would also like to thank Bronwyn
Hegarty, for gently shepherding the project through the many hurdles
encountered and Leigh Blackall, also in the Education Development Unit
at the Otago Polytechnic, for helpful discussions and advice. Many, many
thanks also to Sunshine Blackall for her skills in formatting the
diagrams and designing the artwork accompanying each chapter. The high
quality of this work would not have been possible without the financial
assistance of the Otago Polytechnic CAPEX fund. Thanks are also due to
Jeanette O\'Fee, my Head of School, for encouragement throughout the
project and to Keith Allnatt and Jan Bedford for proofreading and
reviewing. Finally I would like thank Peter and Kate who have patiently
suffered my \"unavailability\" as I tapped my evenings and weekends away
on the computer. *Ruth Lawson*


# Anatomy and Physiology of Animals/Table of contents

**Table of Contents for Print Version**

## Chapter 1 Chemicals

Page Number

14 Objectives

14 Elements and atoms

15 Compounds and molecules

16 Chemical reactions

16 Ionisation

16 Organic and inorganic compounds

17 Carbohydrates

18 Fats

19 Proteins

20 Summary

21 Test Yourself

21 Websites

22 Answers

## Chapter 2 Classification

23 Objectives

24 Naming and classifying animals

24 Naming animals

24 Classification of living organisms

25 The animal kingdom

26 The classification of vertebrates

27 Summary

28 Test yourself

28 Websites

29 Answers

## Chapter 3 The Cell

30 Objectives

30 The cell

32 The plasma membrane

32 How substances move across the plasma membrane

32 The cytoplasm

32 Diffusion

33 Osmosis

35 Active transport

37 Golgi apparatus

38 Lysosomes

38 Microfilaments and microtubules

38 The nucleus

38 Chromosomes

39 Cell division

40 Summary

41 Test yourself

42 Websites

43 Answers

## Chapter 4 Body Organisation

44 Objectives

44 The organisation of animal bodies

45 Epithelial tissues

47 Connective tissues

49 Muscle tissues

50 Nervous tissues

50 Vertebrate bodies

50 Body cavities

51 Organs

51 Generalised plan of the mammalian body

52 Body systems

53 Homeostasis

53 Directional terms

54 Summary

55 Test yourself

56 Websites

57 Answers

## Chapter 5 The Skin

58 Objectives

59 The skin

60 Skin structures made of keratin

60 laws and nails

60 Horns and antlers

61 Hair

62 Feathers

63 Skin glands

64 The skin and sun

64 The skin and temperature regulation

66 Summary

66 Test yourself

67 Websites

67 Answers

## Chapter 6 The Skeleton

69 Objectives

70 The vertebral column

71 The skull

71 The ribs

72 The forelimb

73 The hindlimb

74 The girdles

74 Categories of bones

74 Bird skeletons

75 The structure of long bones

76 Compact bone

77 Spongy bone

77 Bone growth

77 Broken bones

78 Joints

78 Common names of joints

79 Locomotion

79 Summary

80 Test yourself

81 Websites

82 Answers

## Chapter 7 Muscles

83 Objectives

83 Muscles

83 Smooth muscle

84 Cardiac muscle

84 Skeletal muscle

84 Structure of a muscle

85 Summary

85 Test yourself

86 Websites

87 Answers

## Chapter 8 Cardiovascular System

**Blood**

89 Objectives

89 Plasma

90 Red blood cells

91 White blood cells

92 Platelets

92 Transport of oxygen

92 Carbon monoxide poisoning

92 Transport of carbon dioxide

92 Transport of other substances

93 Blood clotting

93 Serum and plasma

93 Anticoagulants

93 Haemolysis

94 Blood groups

94 Blood volume

94 Summary

95 Test Yourself

95 Websites

96 Answers

**The Heart** 96 Objectives

97 The heart

97 Valves

98 The heartbeat

98 Cardiac muscle

99 Control of the heartbeat

99 The coronary vessels

**Blood circulation**

Objectives

102 Blood circulation

103 Arteries

103 The pulse

103 Capillaries

104 The formation of tissue fluid and lymph

104 Veins

105 Regulation of blood flow

105 Oedema and fluid loss

105 The spleen

106 Important blood vessels

106 Blood pressure

107 Summary

107 Test yourself

108 Websites

108 Answers

## Chapter 9 Respiratory System

109 Objectives

109 Respiratory system

110 Diffusion and transportation of oxygen

111 Diffusion and transportation of carbon dioxide

111 The air passages

111 The lungs and pleural cavities

112 Collapsed lung

112 Breathing

112 Inspiration

112 Expiration

113 Lung volumes

113 Composition of air

113 The acidity of the blood and breathing

114 Breathing in birds

114 Summary

114 Test yourself

115 Websites

116 Answers

## Chapter 10 Lymphatic System

117 Objectives

117 Lymph and the lymphatic system

119 Other organs of the lymphatic system

119 Summary

120 Test yourself

121 Websites

121 Answers

## Chapter 11 The Gut and Digestion

122 Objectives

122 The gut and digestion

122 Herbivores

122 Carnivores

122 mnivores

122 Treatment of food

124 The gut

125 Mouth

125 Teeth

126 Types of teeth

126 Dental formula

127 Oesophagus

128 Stomach

128 Small intestine

129 The rumen

129 Large intestine

130 Functional caecum

130 Gut of birds

131 Digestion

131 The liver

133 Summary

134 Test yourself

134 Websites

135 Answers

## Chapter 12 Urinary system

137 Objectives

137 Haemostasis

138 Water in the body

138 Maintaining water balance

139 Excretion

139 The kidneys and urinary system

140 Kidney tubules or nephrons

140 Processes occurring in the nephron

141 The production of concentrated urine

142 Diabetes and the kidney

143 Other functions of the kidney

143 Normal urine

143 Abnormal ingredients of urine

144 Excretion in birds

144 Summary

144 Test yourself

146 Websites

146 Answers

## Chapter 13 Reproductive System

148 Objectives

148 Reproductive system

149 Fertilisation

149 Sexual reproduction mammals

149 The male reproductive organs

150 The testes

151 Semen

151 Accessory glands

151 The penis

151 Sperm

152 The female reproductive organs

153 Ovaries

153 The ovarian cycle

153 The ovum

154 The oestrous cycle

154 Signs of oestrous or heat

155 Breeding seasons or breeding cycles

155 Fertilisation

156 Development of the morula and blastocyst

156 Implantation

156 Pregnancy

157 Hormones during pregnancy

157 Pregnancy testing

157 Gestation period

158 Signs of imminent birth

158 Labour

158 Adaptations of the foetus

158 Milk production

159 Summary

160 Test Yourself

161 Websites

162 Answers

## Chapter 14 Nervous System

164 Objectives

165 Coordination

165 The nervous system

165 The neuron

166 Connections between neurons

167 Reflexes

167 Conditioned reflexes

167 Parts of the nervous system

168 The central nervous system

168 The brain

169 The forebrain

170 The cerebellum

170 The spinal cord

170 The peripheral nervous system

171 Spinal nerves

171 The autonomic nervous system

172 Summary

173 Test Yourself

174 Websites

174 Answers

## Chapter 15 The Senses

176 Objectives

176 The sense organs

177 Touch and pressure

177 Pain

177 Temperature

177 Awareness of limb position

177 Smell

178 Taste

178 Sight

179 The structure of the eye

180 How the eye sees

181 Colour vision in animals

181 Binocular vision

182 Hearing

183 How the ear hears

183 Balance

184 Summary

185 Test Yourself

185 Websites

186 Answers

## Chapter 16 Endocrine System

188 Objectives

188 The endocrine system

189 Endocrine organs and hormones

189 The pituitary gland and hypothalamus

190 The pineal gland

190 The thyroid gland

191 The parathyroid gland

191 The adrenal gland

192 The pancreas

192 The ovaries

192 The testes

192 Summary

194 Test Yourself

194 Websites

194 Answers

**Homeostasis and Feedback Control**

194 Summary of homeostatic mechanisms

194 Temperature control

194 Water balance

195 Maintenance of blood volume after blood loss

196 Acid/base balance

197 Summary

## Glossary