# Anatomy and Physiology of Animals/The Skin

!original image by
Fran-cis-ca cc
by{width="400"}

The skin is the first of the eleven body systems to be described. Each
chapter from now on will cover one body system.

The skin, sometimes known as the **Integumentary System** is, in fact,
the largest organ of the body. It has a complex structure, being
composed of many different tissues. It performs many functions that are
important in maintaining homeostasis in the body. Probably the most
important of these functions is the control of body temperature. The
skin also protects the body from physical damage and bacterial invasion.
The skin has an array of sense organs which sense the external
environment, and also cells which can make **vitamin D** in sunlight.

The skin is one of the first systems affected when an animal becomes
sick so it is important for anyone working with animals to have a sound
knowledge of the structure and functioning of the skin so they can
quickly recognize signs of disease.

## Objectives

After completing this section, you should know:

-   the general structure of the skin
-   the function of the keratin deposited in the epidermis
-   the structure and function of keratin skin structures including
    calluses, scales, nails, claws, hoofs and horns
-   that antlers are not made either of keratin or in the epidermis
-   the structure of hairs
-   the structure of the different types of feathers and the function of
    preening
-   the general structure and function of sweat, scent, preen and
    mammary glands
-   the basic functions of the skin in sensing stimuli, temperature
    control and production of vitamin D
-   the mechanisms by which the skin regulates body temperature

## The Skin

The skin comes in all kinds of textures and forms. There is the dry
warty skin of toads and crocodiles, the wet slimy skin of fish and
frogs, the hard shell of tortoises and the soft supple skin of snakes
and humans. Mammalian skin is covered with hair, that of birds with
feathers, and fish and reptiles have scales. Pigment in the skin, hairs
or feathers can make the outer surface almost any color of the rainbow.

`   Skin is one of the largest organs of the body, making up 6-8% of the total body weight. It consists of two distinct layers. The top layer is called the `**`epidermis`**` and under that is the '''dermis'`

The epidermis is the layer that bubbles up when we have a blister and as
we know from this experience, it has no blood or nerves in it. The cells
at the base of the epidermis continually divide and push the cells above
them upwards. As these cells move up they die and become the dry flaky
scales that fall off the skin surface. The cells in the epidermis die
because a special protein called **keratin** is deposited in them.
Keratin is an extremely important substance for it makes the skin
waterproof. Without it, land vertebrates like reptiles, birds, and
mammals would, like frogs, be able to survive only in damp places.

## Skin Structures Made Of Keratin

### Claws, Nails and Hoofs

Reptiles, birds, and mammals all have nails or claws on the ends of
their toes. They protect the end of the toe and may be used for
grasping, grooming, digging or in defense. They are continually worn
away and grow continuously from a growth layer at their base (see
diagram 5.2).

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

Diagram 5.2 - A carnivore's claw

**Hoofs** are found in sheep, cows, horses etc. otherwise known as
**ungulate mammals**. These are animals that have lost toes in the
process of evolution and walk on the "nails" of the remaining toes. The
hoof is a cylinder of horny material that surrounds and protects the tip
of the toe (see diagram 5.3).

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

Diagram 5.3 - A horse's hoof

### Horns And Antlers

True horns are made of keratin and are found in sheep, goats, and
cattle. They are never branched and, once grown, are never shed. They
consist of a core of bone arising in the dermis of the skin and are
fused with the skull. The horn itself forms as a hollow cone-shaped
sheath around the bone (see diagram 5.4).

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

Diagram 5.4 - A horn

The **antlers** of male deer have quite a different structure. They are
not formed in the epidermis and do not consist of keratin but are entire
of bone. They are shed each year and are often branched, especially in
older animals. When growing they are covered in the skin called
**velvet** that forms the bone. Later the velvet is shed to leave the
bony antler. The velvet is often removed artificially to be sold in Asia
as a traditional medicine (see diagram 5.5).

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

Diagram 5.5 - A deer antler

Other animals have projections on their heads that are not true horns
either. The horns on the head of giraffes are made of bone covered with
skin and hair, and the 'horn' of a rhinoceros is made of modified and
fused hair-like structures.

### Hair

Hair is also made of keratin and develops in the epidermis. It covers
the body of most mammals where it acts as an insulator and helps to
regulate the temperature of the body (see below). The color in hairs is
formed from the same pigment, **melanin** that colors the skin. Coat
color may help camouflage animals and sometimes acts to attract the
opposite sex.

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

Diagram 5.6 - A hair

Hairs lie in a **follicle** and grow from a **root** that is well
supplied with blood vessels. The hair itself consists of layers of dead
keratin-containing cells and usually lies at a slant in the skin. A
small bundle of smooth muscle fibers (the **hair erector muscle**) is
attached to the side of each hair and when this contracts the hair
stands on end. This increases the insulating power of the coat and is
also used by some animals to make them seem larger when confronted by a
foe or a competitor(see diagram 5.6).

The whiskers of cats and the spines of hedgehogs are examples of special
types of hairs.

### Feathers

The lightness and stiffness of keratin is also a key to bird flight. In
the form of feathers, it provides the large airfoils necessary for
flapping and gliding flight. In another form, the light fluffy down
feathers, also made of keratin, are some of the best natural insulators
known. This superior insulation is necessary to help maintain the high
body temperatures of birds.

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

Diagram 5.7 - A Contour Feather

Contour feathers are large feathers that cover the body, wings, and
tail. They have an expanded **vane** that provides the smooth,
continuous surface that is required for effective flight. This surface
is formed by **barbs** that extend out from the central shaft. If you
look carefully at a feather you can see that on either side of each barb
are thousands of **barbules** that lock together by a complex system of
hooks and notches. if this arrangement becomes disrupted, the bird uses
its beak to draw the barbs and barbules together again in an action
known as **preening** (see diagram 5.7).

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

Diagram 5.8 - A Down Feather

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

Diagram 5.9 - A Pin Feather

Down feathers are the only feathers covering a chick and form the main
insulation layer under the contour feathers of the adult. They have no
shaft but consist of a spray of simple, slender branches (see diagram
5.8).

Pin feathers have a slender hair-like shaft often with a tiny tuft of
barbs on the end. They are found between the other feathers and help
tell a bird how its feathers are lying (see diagram 5.9).

## Skin Glands

Glands are organs that produce and secrete fluids. They are usually
divided into two groups depending upon whether or not they have channels
or ducts to carry their products away. Glands with ducts are called
**exocrine glands** and include the glands found in the skin as well as
the glands that produce digestive enzymes in the gut. **Endocrine
glands** have no ducts and release their products (hormones) directly
into the bloodstream. The pituitary and adrenal glands are examples of
endocrine glands.

Most vertebrates have exocrine glands in the skin that produce a variety
of secretions. The slime on the skin of fish and frogs is **mucus**
produced by skin glands and some fish and frogs also produce poison from
modified glands. In fact, the skin glands of some frogs produce the most
poisonous chemicals known. Reptiles and birds have a dry skin with few
glands. The **preen gland**, situated near the base of the bird's tail,
produces oil to help keep the feathers in good condition. Mammals have
an array of different skin glands. These include the wax producing,
sweat, sebaceous and mammary glands.

**Wax producing glands** are found in the ears.

**Sebaceous glands** secrete an oily secretion into the hair follicle.
This secretion, known as **sebum**, keeps the hair supple and helps
prevent the growth of bacteria (see diagram 5.6).

**Sweat glands** consist of a coiled tube and a duct leading onto the
skin surface. Their appearance when examined under the microscope
inspired one of the first scientists to observe them to call them
"fairies' intestines" (see diagram 5.1). Sweat contains salt and waste
products like urea and the evaporation of sweat on the skin surface is
one of the major mechanisms for cooling the body of many mammals. Horses
can sweat up to 30 liters of fluid a day during active exercise, but
cats and dogs have few sweat glands and must cool themselves by panting.
The scent in the sweat of many animals is used to mark territory or
attract the opposite sex.

**Mammary glands** are only present in mammals. They are thought to be
modified sweat glands and are present in both sexes but are rarely
active in males (see diagram 5.10). The number of glands varies from
species to species. They open to the surface in well-developed nipples.
Milk contains proteins, sugars, fats and salts, although the exact
composition varies from one species to another.

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

Diagram 5.10 - A Mammary Gland

## The Skin And Sun

A moderate amount of UV in sunlight is necessary for the skin to form
**vitamin D**. This vitamin prevents bone disorders like rickets to
which animals reared indoors are susceptible. Excessive exposure to the
UV in sunlight can be damaging and the pigment **melanin**, deposited in
cells at the base of the epidermis, helps to protect the underlying
layers of the skin from this damage. Melanin also colors the skin and
variations in the amount of melanin produce colors from pale yellow to
black.

### Sunburn And Skin Cancer

Excess exposure to the sun can cause sunburn. This is common in humans,
but light skinned animals like cats and pigs can also be sunburned,
especially on the ears. Skin cancer can also result from excessive
exposure to the sun. As holes in the ozone layer increase exposure to
the sun's UV rays so too does the rate of skin cancer in humans and
animals.

## The Dermis

The underlying layer of the skin, known as the dermis, is much thicker
but much more uniform in structure than the epidermis (see diagram 5.1).
It is composed of loose connective tissue with a felted mass of
**collagen** and **elastic fibres**. It is this part of the skin of
cattle and pigs etc. that becomes commercial leather when treated, The
dermis is well supplied with blood vessels, so cuts and burns that
penetrate down into the dermis will bleed or cause serious fluid loss.
There are also numerous nerve endings and touch receptors in the dermis
because, of course, the skin is sensitive to touch pain and temperature.

When looking at a section of the skin under the microscope you can see
hair follicles, sweat, and sebaceous glands dipping down into the
dermis. However, these structures do not originate in the dermis but are
derived from the epidermis.

In the lower levels of the dermis is a layer of fat or **adipose
tissue** (see diagram 5.1). This acts as an energy store and is an
excellent insulator especially in mammals like whales with little hair.

## The Skin And Temperature Regulation

Vertebrates can be divided into two groups depending on whether or not
they control their internal temperature. Amphibia (frogs) and reptiles
are said to be"**cold blooded**" (**poikilothermic**) because their body
temperature approximately follows that of the environment. Birds and
mammals are said to be **warm blooded (homoiothermic**) because they can
maintain a roughly constant body temperature despite changes in the
temperature of the environment.

Heat is produced by the biochemical reactions of the body (especially in
the liver) and by muscle contraction. Most of the heat loss from the
body occurs via the skin. It is therefore not surprising that many of
the mechanisms for controlling the temperature of the body operate here.

### Reduction Of Heat Loss

When an animal is in a cold environment and needs to reduce heat loss
the erector muscles contract causing the hair or feathers to rise up and
increase the layer of insulating air trapped by them.

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

Diagram 5.11a) Hair muscle relaxed\...\...\...\...\...Diagram 5.11b)
Hair muscle contracted

Heat loss from the skin surface can also be reduced by the contraction
of the abundant blood vessels that lie in the dermis. This takes blood
flow to deeper levels, so reducing heat loss and causing pale skin (see
diagram 5.12a).

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

Diagram 5.12a) Reduction of heat loss by skin

Shivering caused by twitching muscles produces heat that also helps
raise the body temperature.

### Increase Of Heat Loss

There are two main mechanisms used by animals to increase the amount of
heat lost through the skin when they are in a hot environment or high
levels of activity are increasing internal heat production. The first is
the expansion of the blood vessels in the dermis so blood flows near the
skin surface and heat loss to the environment can take place. The second
is by the production of sweat from the sweat glands (see diagram 5.12b).
The evaporation of this liquid on the skin surface produces a cooling
effect.

The mechanisms for regulating body temperature are under the control of
a small region of the brain called the **hypothalamus**. This acts like
a thermostat.

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

Diagram 5.12b) - Increase of heat loss by skin

### Heat Loss And Body Size

The amount of heat that can be lost from the surface of the body is
related to the area of skin an animal has in relation to the total
volume of its body.

Small animals like mice have a very large skin area compared to their
total volume. This means they tend to lose large amounts of heat and
have difficulty keeping warm in cold weather. They may need to keep
active just to maintain their body temperature or may hibernate to avoid
the problem.

Large animals like elephants have the opposite problem. They have only a
relatively small skin area in relation to their total volume and may
have trouble keeping cool. This is one reason that these large animals
tend to have sparse coverings of hair.

## Summary

-   Skin consists of two layers: the thin **epidermis** and under it the
    thicker **dermis.**
-   The **Epidermis** is formed by the division of base cells that push
    those above them towards the surface where they die and are shed.
-   **Keratin**, a protein, is deposited in the epidermal cells. It
    makes skin waterproof.
-   Various skin structures formed in the epidermis are made of keratin.
    These include claws, nails, hoofs, horn, hair, and feathers.
-   Various **Exocrine Glands** (with ducts) formed in the epidermis
    include sweat, sebaceous, and mammary glands.
-   **Melanin** deposited in cells at the base of the epidermis protects
    deeper cells from the harmful effects of the sun.
-   The **Dermis** is composed of loose connective tissue and is well
    supplied with blood.
-   Beneath the dermis is insulating **adipose tissue**.
-   Body Temperature is controlled by sweat, hair erection, dilation,
    and contraction of dermal capillaries and shivering.

## Worksheet

Use the Skin Worksheet to
help you learn all about the skin.

## Test Yourself

Now use this Skin Test Yourself to see how much you have learned and
remember.

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1\. The two layers that form the skin are the a)
_   _ and b) _   _

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:   a)epidermis
:   b)dermis

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2\. The special protein deposited in epidermal cells to make them
waterproof is:

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keratin

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3\. Many important skin structures are made of keratin. These include:

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hair,nails,foot pads,feathers,scales on reptiles

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4\. Sweat, sebaceous and mammary glands all have ducts to the outside.
These kind of glands are known as:_    _

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exocrine

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5\. What is the pigment deposited in skin cells that protects underlying
skin layers from the harmful effects of the sun?

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melanin

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6\. How does the skin help cool an animal down when it is active or in a
hot environment?

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Panting and secretion from sweat glands.

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7\. Name two mechanisms by means of which the skin helps prevent heat
loss when an animal is in a cold environment.

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:   a.shivering
:   b.contraction of blood vessels

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/Test Yourself Answers/

## Websites

-   <http://www.auburn.edu/academic/classes/zy/0301/Topic6/Topic6.html>
    Comparative anatomy

Good on keratin skin structures - hairs, feathers, horns etc.

-   <http://www.olympusmicro.com/micd/galleries/brightfield/skinhairymammal.html>
    Hairy mammal skin

All about hairy mammalian skin.

-   <http://www.earthlife.net/birds/feathers.html> The wonder of bird
    feathers

Fantastic article on bird feathers with great pictures.

-   <http://en.wikipedia.org/wiki/Skin> Wikipedia

Wikipedia on (human) skin. Good as usual, but more information than you
need.

## Glossary

-   Link to
    Glossary

