integumentary system : skin skin is the largest organ in the body. also included are the accessory...
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Integumentary System :Skin
skin is the largest organ in the body. Also included are the
accessory organs: hair, sweat glands, sebaceous glands, nails
and variations
Skin:The study of skin is dermatology.
The skin has a variety of functions including:*barrier against invasion of micro-organisms*protects underlying structures from injury
*prevents the body from drying*inhibits excess water intake (from aquatic species)
*regulates temperature*receptor for sensations of touch, heat, cold, pressure
and pain.*disposal of waste products
The body produces heat by metabolizing ingested food and the
amount of heat produced is the result of the amount of work done
by the muscles.Most of body heat is lost by:
Conduction: heat energy passes by direct contact through a non-moving medium.
Eg: when an animal lies on a cold surface it loses heat directly to that surface.
Or when an animal is lying on a surgery table the heat is lost directly to the surface of the
table and through the incision made
Convection: heat transfer occurs in a gas or liquid by the
circulation of currents from one region to another. This is the heat loss responsible for the
wind chill factor
Evaporation; heat is lost as perspiration or other fluids on
the skin. Air currents can increase evaporative heat loss
through convection.
Most of the remainder of heat loss occurs through mucous
membranes and lesser amounts lost through digestive and urinary
tracts
Composition of skinthe skin is composed of two
layers:the epidermis: outer, thinner layerthe dermis: the thicker inner layer
The Epidermis: is made up of stratified squamous
epithilial tissue,the layers from the dermis
outward are
Stratum germinativum: (basal layer)
The cells in this inner most layer multiply continuously to
compensate for the constant loss of cells from the surface of the epidermis. These cells push upward into each layer and
eventually die.
Stratum spinosum:(spinous layer)
The cells begin to shrink and draw apart. This layer is only
one or two cells thick. It is thicker on parts of the body that endure rugged wear (footpads)
Stratum granulosum:(granular layer)
The cells in this layer contain granules visible in the cytoplasm. This layer may not be present in
some areas of thin skin.Keratinization or cornification
packs the cells with fibrous protein called keratin
Stratum lucidum:(clear layer)
This layer is made up of closly packed clear cells. This layer is
found in tissue that endure rugged wear.
Stratum corneum:(horny layer)
This layer is made of flat lifeless cornified cells. They appear as
overlapping dry scales that make up the outer skin layer. If unbroken, they will prevent the entrance of microorganisms.
Keratinization gives the special epidermal parts (hooves, horns, beaks)
their strength.
Skin color is determined by the amount of melanin in the
stratum germinativum layer. Heredity, the sun and some
hormones affect the skin color
The absence of normal pigmentation is called albinism.
True albinism prevents pigmentation of the skin, hair and
eyes. This condition may be associated with other lethal traits
as well as a lack of protection from the sun
Skin color can also be affected by the blood supply. Eg. Skin color can take on a blue hue
(cyanosis) when oxygen supply is compromised
The dermis (corium) is made up of dense, fibrous connective
tissue containing blood vessels and nerves. Small involuntary
muscles (arrector pili) are attached to the hair follicles.
When these muscles contract, the hairs stiffen. This increases the animals ability to insulate itself
against cold.
The subcutaneous layer under the dermis is the hypodermis or
subcutis. It consists of loose connective tissue and adipose
tissue.
Primary hairs are also called guard hairs and these make up
the top coat. These hairs are stiff with a smooth appearance. This arrangement allows rain to run off the coat preventing chilling
of the animal
Secondary hairs are also called wool hairs and make up the
undercoat. These hairs are soft, thin and wavy.
Tactile hair are usually thicker and longer than primary hair and
is most often found about the face. These hairs grow deep
from the hypodermis or superficial muscle layer and act
as a touch receptor.
Hair develops from the hair papilla, located at the base of the
hair follicle. Cells at the base increase, push upward and
keratinize forming the visible hair shaft. Hair follicles are positioned at 30 – 60 degree
angle to the skin.
The two arrangements for hair growth:
simple: primary hairs grow from separate follicular openings
complex: two to five guard hairs erupt from a single follicle. And
are surrounded by clusters of wool hairs
Hair color is important for thermal regulation. In general, lighter color coats are cooler.Animal hair grows in cycles.
Factors affecting the shedding cycle: temperature, nutrition, hormones, genetics, state of
health.
Sweat glands are excretory organs of the skin that cool the
body.Apocrine sweat glands are found throughout the body and secrete a strong smelling substance in the
hair
Eccrine sweat glands are coiled tubular structures embedded in
the dermis and produce a watery sweat. These glands are found in limited areas usually the footpads
and nostrils
The sebaceous glands secrete a substance called sebum which
lubricates the skin and hair, waterproofs the coat, gives the
coat a glossy sheen, increases the spread of sweat, slows bacterial
growth, and serves as a territorial marker.
Examples of these glands are:
Circumoral glands:These glands are found in the
lips of cats. During grooming or when a cat rubs it’s head against
and object, the secretion is deposited to mark territory
Horn glands:These glands produce a musk odor that attracts females to
males during breeding season. They are located caudo-medially
to the horn base.
Anal sacs:These are cutaneous pouches that express a foul smelling fluid that
is expressed during defication and is used as a territorial
marker.
Ceruminous glands:These are classified as modified
sweat glands. They are located in the external ear canal. It excretes a yellowish waxy substance called
cerumen (earwax)
Nails, Claws and Hooves.All three of these structures have the following parts in common:
*wall*sole*pad
In primates the nail is the wall and it grows from the epidermis. The sole is located beneath the nail at the open end. The pad
corresponds with the finger tips
In carnivores the claw is the wall, which has been compressed
laterally. The sole is the underside of the wall and is flaky in texture. The pad corresponds
to the foot pads
Plantigrade animals have well developed footpads.
Digitigrade animals include cats and dogs. Only the digital and metacarpal pads make ground
contactUngulates include hoofed animals