2022 Skin Anatomy Lecture

SKIN ANATOMY
AND FUNCTION

Course Objectives

At the completion of this course, the participants
will be able to:

Properly define the functional characteristics of the
three layers of skin.
Describe the form and function of the following
somatosensory receptors: mechanoreceptors,
temperature receptors, and Nociceptors.
Correctly identify the proper testing procedures for
determining the status of Mechanoreceptors,
thermoreceptors, and Nociceptors.
Describe common skin conditions and impairments
that can affect the skin.
Discuss the affect that the aging process has on skin
integrity.

Skin is our largest organ. If the skin of a typical 150-pound (68-
kilogram) adult male were stretched out flat, it would cover about 2
square yards (1.7 square meters) and weigh about 9 pounds (4
kilograms). It shields the body against heat, light, injury, and
infection. The skin also helps regulate body temperature, gathers
sensory information from the environment, stores water, fat, and
vitaminD, and plays an important role in the immune system
protecting us from disease.In addition to this, it has a remarkable
regenerative quality.

Integument

The form that the skin takes will depend
upon its function.

– Color
– Thickness
– Texture
– Redundancy
– Integrity

To really understand the skin, we need to look at it from the perspective of form
and function. The form that the skin takes will depend upon its function. For
example, the color, thickness and texture of the skin differ with each part of the
body. Some areas of the body have loose, redundant skin to allow for joint range
of motion whereas other parts are less redundant to provide stability (such as our
torso and scalp).

Types of Skin

• Hairy Skin • Glabrous Skin

Thinner Hairless
Most common Thicker

Skin is composed of two basic types: thin and hairy (the most common) and thick
and hairless (associated with those parts of the body that are used heavily and
are exposed to activities that can cause different forms of friction, such as our
hands and feet).

Physiological Function

• Regulation
of the
body’s

temperature

Physiologically, the skin serves several functions. It regulates the body’s
temperature through blood flow and sweat. During exercise, for example, the
evaporation of sweat will lower body temperature. In cooler environments,
production of sweat will decrease and blood flow increase to adjust body
temperature.

Physiological Function

• Defensive Barrier to
Protect Against:

– Physical Injury

The skin is a defensive barrier that protects underlying tissues against physical
injury…

Physiological Function

• Defensive Barrier to Protect Against:

Physical Injury
Microorganism Invasion
Dehydration

. microorganism invasion and dehydration….

Physiological Function

• Defensive Barrier to Protect Against:

Physical Injury
Microorganism Invasion
Dehydration
Ultraviolet Radiation

and ultraviolet light radiation.

Physiological Function

• Sensory Organ

The skin is a sensory organ that contains specialized somatosensory receptors
that detect and locate stimuli related to temperature, touch, pressure, and pain.

Physiological Function

• Skin acts as a
Blood Reserve

Finally, it is a blood reserve that incorporates extensive networks of blood
vessels (between 8-10% of total blood flow in resting adults). With moderate
exercise, blood flow will increase in the dermis in an effort to help dissipate heat
from the body. In strenuous exercise, though, blood vessels will constrict to
circulate more blood to contracting muscles.

Fingernails

Fingernails are a type of modified skin. Nails protect the sensitive tips of fingers
and toes. Human nails aren't necessary for living, but they do provide support for
the tips of the fingers and toes, protect them from injury, and aid in picking up
small objects. Without them, we'd have a hard time scratching an itch or untying
a knot. Nails can be an indicator of a person's general health, and illness often
affects their growth.

Layers of Skin

Epidermis

– Keratinocytes
– Melanocytes
– Langerhan’s Cells
– Merkel Cells

The skin is composed of three layers. Each of these layers varies in form and
function. The outer layer is the epidermis. It is a tough protective layer devoid of
a nerve and blood supply that is structured to prevent loss of essential body
fluids and to protect the body against invasion of microorganisms that can be
found in our environment. Without this layer, we would dehydrate rapidly due to
water evaporating into the environment. The thickness of the epidermis varies at
different areas of the skin. The thinnest is found in the eyelids at just 0.05mm
and the thickest is 1.5mm on the palms and soles of the feet. Although the
epidermis is quite thin, it is made up of five layers. The deepest layer is the
stratum basale whose role is to germinate new cells. This layer contains
sensitive touch receptors. At the stratum spinosum layer, cells begin to attach
themselves to one another more tightly. The stratum granulosum begins the
process of water proofing cells so that the skin can become more water resistant.
Here the cells begin to degenerate and die. The stratum lucidum, normally found
only in the hands and soles of the feet, transform the cells to keratin. The
stratum corneum is the most superficial layer that consist of about 25-30 layers of
dead cells completely filled with keratin. These cells are continuously shed and
replaced by cells from deeper strata. The stratum corneum serves as an effective
barrier against light and heat waves, bacteria, and many chemicals from the
environment.

Epidermis

The epidermis is made up of specialized cells. The keratinocyte is the most
prominent cell in the epidermis and is responsible for producing keratin and
creation of an extracellular water barrier. Melanocytes comprise about 10% of the
cells in the epidermis. These cells are responsible for skin color. The
Langerhan’s cell initiates cutaneous contact hypersensitivity reactions that help
white blood cells fight off infection. Finally, the Merkel cell is located at the base
of the epidermis and is a very sensitive mechanoreceptor and is responsible for
identifying tactile sensations.

Dermis

• Thicker than the
Epidermis

• 2 Layers:

Papillary
Reticular

The thicker dermis lies below the epidermis. It also varies in thickness
depending on the location and the foundation of the skin, 0.1 mm on the eyelid
and 4.0mm on the back. It contains nerve ending, sweat glands, oil glands, and
hair follicles. It also has an extensive blood supply system. There are two layers
of dermis: the more superficial papillary layer and the deeper reticular layer. The
papillary layer has less densely packed collagen fibers and more elastic fibers
structured in an irregular network. This allows for better movement and rebound
against external forces. Nerve endings are concentrated in this area of the
dermis. The reticular layer is considerably thicker and has more course elastic
collagenous fibers to provide greater stability. This layer contains the Ruffini end
organs, blood vessels and sweat glands.



Hypodermis

• Subcutaneous Layer

Anchor for dermis
Aerolar
Adipose

The hypodermis is a subcutaneous layer of aerolar (tissue that lubricants and
nourishes epithelial tissue) and adipose (fat) tissues. The dermis will extend
down into the hypodermis and anchor the skin to it. The hypodermis, in turn,
anchors to underlying tissues and organs. The subcutaneous layer is composed
of fat and connective tissue and houses larger blood vessels and nerves. The
function is to regulate temperature of the skin and maintain the body’s core
temperature. The thickness of this layer varies widely between people. It also
acts as a shock absorber to protect interior structures.

Somatosensory Receptors

The skin is an extensive sensory receptor for temperature, pain, touch and
pressure. The brain’s ability to discriminate between the quality and type of
stimulus is dependent on the specificity of the receptor for the specific stimuli.
For example, vision responds to the stimuli of light, touch to pin prick, and
proprioception of joint movement or displacement. Specialized nerves can sense
and transmit heat, pain, and other noxious sensations. When these receptors
are not working properly, sensations such as numbness, pins-and-needles, pain,
tingling, or burning may be felt.

Pathways
for

Touch

Spinal nerves have motor fibers and sensory fibers. The motor fibers innervate
certain muscles, while the sensory fibers innervate certain areas of skin. A skin
area innervated by the sensory fibers of a single nerve root is known as a
dermatome. Although slight variations do exist, dermatome patterns of
distribution are relatively consistent from person to person.

When a stimulus is detected and an increase in stimulus intensity reaches
beyond the threshold for an action potential by a particular receptor, that impulse
translates to an increased rate of firing of the afferent fiber, thus sending
information to the central nervous system. The receptors detect both stimulus
and intensity. There are half a dozen receptors in the skin which are considered
as somatosensory receptors.

Somatosensory Receptors

Free Nerve Endings • Merkel’s Disks
Krause End Bulb • Ruffini’s End Organs
Meissner Corpuscles • Pacinian Corpuscles

The most numerous sensory receptors are free nerve endings in the papillary
layer of the dermis. Networks of free nerve endings surround most hair follicles
attached to their outer root and are very sensitive. The Meissner corpuscles are
touch receptors, particularly found in the papillary of hairless skin, such as the
lips and palmar surfaces of the fingers and toes. Merkels discs are touch
receptors which are located in the dermis layer over the palms of the hands and
soles of the feet. The Rufinni end organs respond to mechanical displacement of
the adjacent collagen fibers that pass through its receptor. The Pacinian
corpuscles are highly sensitive receptors located deep in the dermis and joint
capsules. Krause end bulbs, also called bulbous capsules in the skin contain
sensory nerve endings, which may be mechanoreceptors, but which are also
thought to be thermoreceptors sensitive to cold and activated by temperatures
less than 20 °C. They occur more superficially in the skin than heat receptors and
are rapidly adapting and respond best to vibration rather than to prolonged
pressure.

Types of Sensory Receptors

Adaptation (or accommodation) is a characteristic property of sensory receptors
and describes a progressive decrease in firing frequencies despite maintained
depolarization. The frequency of action potentials from stimulated receptors falls,
although the stimulus is maintained at constant strength.
Sensory receptors can be basically categorized into two types: slow (incomplete)
adapting and fast (completely) adapting. The slow adapting fibers continue to
fire for a prolonged period during the maintained stimulus. The fast adapting
fiber receptors will cease firing after the initial stimulus has been applied.

Mechanoreceptors

Mechanoreceptors respond to pressure on the skin. Near the surface are
Merkel’s disks and Meissner’s corpuscles. Both have small receptive fields but
Meissner's corpuscles are rapidly adapting while Merkel's discs are slowly
adapting mechanoreceptors. Deeper in the skin are Ruffini’s end organs and
Pacinian Corpuscles. Both have large receptive fields. Ruffini's are slowly
adapting and Pacinian's are rapidly adapting mechanoreceptors.

Temperature Receptors

Temperature receptors include free nerve endings of small unmyelinated axons
(sometimes called hot fibers). Their firing rate increases when skin
temperature is above 35 degrees Centigrade. Cold fibers are free nerve endings
of small myelinated axons. They increase their firing rate if skin temperature falls
below 35 degrees Centigrade and also when temperature rises above 45
degrees Centigrade. The latter response may account for paradoxical cold (e.g.,
shivering and goose bumps in a hot bathtub).

Pain

Pain is caused by a stimuli (chemical, mechanical, thermal, or electrical) of such
an intensity that it can produce tissue damage. Pain is a high threshold
sensation. The nociceptors (pain receptors) are free nerve endings. Cutaneous
pain may be sharp and localized, or dull and diffuse. A painful stimulus causes at
first a sharp pain, followed by dull aching pain. Reflex withdrawal movements
also occur, along with an associated increase in heart rate and blood pressure.
Fast, sharp (pricking) pain is mediated by nociceptors innervated by group A-
delta thick myelinated nerve fibers which transmit pain impulses at a velocity of
20 meter/second. Slow chronic (dull, aching or burning) pain is mediated by
nociceptors innervated by C-fibers, thin unmyelinated nerve fibers that conduct
pain at a low velocity of 1 meter/second.

Testing Somatosensory
Receptors

Patients are typically referred to occupational or physical therapy to evaluate the
status of their sensation. Being proficient at performing a sensory test is certainly
important. However, knowing what somatosensory receptors are responsible for
which sensation will help to define the intervention strategy. Assessing sensation
involves testing receptors for constant pressure, vibration and skin movement,
temperature and pain.

Constant Skin Pressure

• Found in all type of skin
• Slow Adapting Fibers

Merkel Cell
Rufinni End Organ

• Test:

2 Point Discrimination
Deep Pressure

Receptors for constant skin pressure can be found on hairy skin, located at
the epidermis-dermis junction. On glabrous (or hairless) skin, they are located at
the base of the skin ridge (the forming of a fingerprint). Merkel cell translates
mechanical energy into a nerve impulse – telling the brain of the duration and
magnitude of the stimulus.
These responds to constant pressure and are slowly adapting. The Ruffini end
organ is a slowly adapting fiber. It sends neural signals in response to experience
of pressure and constant touch. They are thought to be responsible for the ability
to rate cutaneous pressure on a scale of intensity. To test the status of these
receptors, an aesthesiometer would be used to measure static two point
discrimination.

Vibration and Skin Movement

Found in both types of skin
Quick Adapting Fibers
No contribution to information about deep
pressure or constant touch

Receptors responding to Vibration and Skin Movement can be found in both
hairy and glabrous skin. Unlike slowly adapting mechanoreceptors, quick
adapting fibers remain silent during skin deformation. Receptors found in hairy
skin are the typical hair follicle. It is surrounded by a network of nerve terminals
of large myelinated axons. These rapidly adapting fibers are very sensitive to
moving or vibratory stimuli. They do not contribute to information about deep
pressure or constant touch.

Vibration and Skin Movement

• Pacinian Corpuscle

Quickly Adapting Fiber

Deep Pressure (constant 2PD)
Vibrations around 250 Hz

• Meissner Corpuscle

Quickly Adapting Fiber

Light Touch (moving 2 PD)
Vibrations around 40 Hz

In glabrous skin, the pacinian and meissner corpuscles are the active receptors.
The pacinian corpuscle is located in the deep dermis and hypodermis. It is one of
the body’s largest afferent fibers and is a quickly adapting receptor. When
provided constant pressure, it produces and impulse only at the beginning and
the end of the stimulus. Pacinian corpuscles respond to deep pressure and to
vibrations around 250 Hz. The meissner corpuscle is located more superficially
in the papillary layer of the dermis. It responds to light touch and to lower
frequency vibrations of around 40 Hz. It too is a rapidly adapting receptor. The
status of these receptors can be tested using an aesthesiometer to test for
constant pressure, a tuning fork at 250 cps to test vibratory sensation, or a
moving 2 point discrimination test to measure the status of the quickly adapting
fibers.

Free Nerve Endings

• Respond to: • Routine Tests:

Cold – Hot/Clod
Warmth
Heat Pin Prick
Pain Soft Touch
Itch

Free Nerve Endings come in different sizes and are the most numerous of all
receptors in the skin. They are very sensitive mechanoreceptors that alert the
central nervous system with a variety of environmental events. Free nerve
endings are found at the epidermis-dermis junction and respond to stimuli
interpreted as cold, warmth, heat, pain, itch. Testing the status of free nerve
endings can be accomplished by performing tests of hot/cold, pin prick, and soft
touch.

Nociceptors

Nociceptors are free nerve endings that respond to stimuli which pose a threat.
There is an important difference between nociception and pain. Nociception is
the reception of afferent information from specialized sensory receptors to the
CNS to provide information about tissue damage. An example of this would be
your reaction when you touch a hot surface, such as a hot stove or iron. Pain is
a subjective perception of an unpleasant sensation that originates from a
segment of the body.

This is a summary slide of what we have just discussed. For each of the types of
receptors listed on the left side of the table, namely Mechanoreceptors,
Thermoreceptors, and Nociceptors, you will find a corresponding stimulus and
receptor category listed. Mechanoreceptors respond to pressure, such as hair
movement, light to deep pressure and touch. The specific nerves associated
with mechanoreceptors are free nerve endings, meissner and pacinian
corpuscles, Krause end bulbs and merckel’s disks. Thermoreceptors detect cold
and heat. The cold receptors are made up of unmyelinated nerve fibers and the
heat receptors are made up of myelinated nerve fibers. Finally, nociceptors are
free nerve ending found throughout our bodies that can detect pain.

Things That Can Go Wrong
With the Skin

Now that we have an idea of what comprises the skin, lets take a look at what
can go wrong with the skin. We will start by looking at conditions that can be
seen on the surface of the skin.

Things That Can Go Wrong
With the Skin

Dermatitis:
inflammation of the
skin

Atopic Dermatitis

– Eczema

Dermatitis
The term dermatitis refers to any inflammation (swelling, itching, and redness)
possibly associated with the skin. There are many types of dermatitis, including:
Atopic dermatitis or eczema. It's a common, hereditary dermatitis that causes
an itchy rash primarily on the face, trunk, arms, and legs. It commonly develops
in infancy, but can also appear in early childhood. It may be associated with
allergic diseases such as asthma and seasonal, environmental, and food
allergies.

Things That Can Go Wrong
With the Skin

Contact dermatitis. This occurs when the skin comes into contact with an
irritating substance or one that the person is allergic or sensitive to. The best-
known cause of contact dermatitis is poison ivy, but there are many others,
including chemicals found in laundry detergent, cosmetics, and perfumes, and
metals like nickel plating on jewelry, belt buckles, and the back of a snap.

Things That Can Go Wrong
With the Skin

Seborrheic Dermatitis

Seborrheic dermatitis is an oily rash, which appears on the scalp, face, chest,
and back, related to an overproduction of sebum from the sebaceous glands.
This condition is common in infants and adolescents.

Things That Can Go Wrong
With the Skin

Bacterial Skin Infections
Impetigo is a bacterial infection that results in a honey-colored, crusty rash, often
on the face near the mouth and nose.

Things That Can Go Wrong
With the Skin

Cellulitis is an infection of the skin and subcutaneous tissue that typically occurs
when bacteria are introduced through a puncture, bite, or other break in the skin.
The area with cellulitis is usually warm, tender, and has some redness.
Streptococcal and staphylococcal infections are two kinds of bacteria and are
the main causes of cellulitis and impetigo. Certain types of these bacteria are
also responsible for distinctive rashes on the skin, including the rashes
associated with scarlet fever and toxic shock syndrome.

Things That Can Go Wrong
With the Skin

Viral infections. Many viruses cause characteristic rashes on the skin, including
varicella, the virus that causes chickenpox and shingles; herpes simplex, which
causes cold sores; human papillomavirus is the virus that causes warts; and
there are a host of others.

Things That Can Go Wrong
With the Skin

Acne is also called acne vulgaris. Acne is most common in teens. Some degree
of acne is seen in 85% of adolescents, and nearly all teens have the occasional
pimple, blackhead, or whitehead. Acne can also be found in adults as well.

Things That Can Go Wrong
With the Skin

Skin cancer is rare in children and teens, but good sun protection habits
established during these years can help prevent skin cancers such
as melanoma (which is a serious form of skin cancer that can spread to other
parts of the body) later in life, especially among fair-skinned people who sunburn
easily.

Impairment

Peripheral Nervous System

– Disease or Injury
– Fractures or Lacerations
– Diabetes or Burns

Central Nervous System

– Multiple Sclerosis
– Acquired Brain Injuries

Impairment can occur below the skin. Impairment to the sensory system can
occur in either the peripheral or central nervous systems. Disease or injury of the
peripheral nervous system can affect sensory input at the receptor level, such as
with diabetes or burns. Furthermore, injuries can happen along the extremity
that affects the peripheral nerve and can cause sensory changes distally, as
happens with lacerations and some fractures. Also, sensation can be interfered
at the central nervous system by conditions such as multiple sclerosis, acquired
brain injury, and CVA. Being able to correctly identify the source of the sensory
impairment aids in the proper intervention strategy.

Aging Process

•Sunlight: cause of
• Wrinkles
•Dryness
•Age Spots

Sunlight is a major cause of age related changes such as wrinkles, dryness and
age spots.
As the skin ages, it becomes thinner and loses fat, looks less plump and smooth.
The skin loses it elasticity and becomes thinner. Underlying structures, such as
veins and bones, become more prominent. Because of lack of protection due to
the loss subcutaneous fat, the skin can be more easily damaged and take longer
to heal.

Sunlight

• Age Related Changes
• Skin looses elasticity
• Loses subcutaneous

fat

Become thinner
Bones become more
prominent

By age 35, replenishing epidermal cells may take twice as long as during
adolescence. This prolonged turnover affects the skin’s barrier functions and
increases the risk of skin cancer and infection. In older adults, even more
significant skin changes occur. The number of Langerhans cells decreases,
which diminishes inflammatory responses. Decreased epidermal regeneration
and avascular supply alter allergic reactions and delay wound healing. Older
adults may sweat less because of fewer sweat glands. This could result in
hyperthermia.
The skin loses it elasticity and becomes thinner. Underlying structures, such as
veins and bones, become more prominent. Because of lack of protection due to
the loss subcutaneous fat, the skin can be more easily damaged and take longer
to heal.

• Dry Skin

Rough
Scaly
Itchy

• Causes:

Dehydration
Sun Exposure
Smoking
Stress

Many older individuals suffer from dry skin. In this case, it is psoriasis. Skin
feels rough and scaly and becomes quite itchy. The loss of sweat and oil glands
due to aging can aggravate dry skin conditions. Dehydration, sun exposure,
smoking, and stress can all contribute to dry skin. Dry skin conditions can lead to
problems associated with the healing process.

Impairment

Malnutrition

– Increased susceptibility to skin breakdown
– Impairs wound healing process

Dehydration

– Decreased blood volume
– Affects nutrient and oxygen uptake

Malnutrition associated with weight loss and decreased protein levels make
tissues more susceptible to breakdown and impairs wound healing. Individuals
who do not consume enough calories lose adipose tissue and lean body mass.
Poor appetite and low dietary protein intake can place a person at higher risk of
skin breakdown.

Dehydration, which can often accompany malnutrition, also is a risk factor
because reduced blood volume interferes with peripheral circulation and nutrient
and oxygen supply to tissues.

Aging Process

Brain’s ability to interpret sensory input
Typical changes:

– decrease in acuity of many sensations
– decrease in average weight
– degeneration of neurons
– decrease in enzyme production
– depletion of the neuronal dendrites in the

brain

An important consideration to take into account is the influence that the aging
process has on the brain’s ability to interpret somatosensory input. There are
several physiological changes that normally accompany the aging process.
Typical changes include decrease in acuity of many sensations, decrease in
average weight, degeneration of neurons and replacement gliosis, decrease in
the number of enzymes responsible for dopamine or norepinephrine synthesis,
and depletion of the neuronal dendrites in the brain.

Aging Process

Physiological Changes

– a reduction of receptors that can pick up
sensory stimuli

– decreased distance between the nodes of
Ranvier

– degeneration of myelinated fibers in the spinal
cord

Along with the physiological changes associated with age, there are some
anatomical structures that are affected as well. As noted here, there is a
reduction of receptors that can pick up sensory stimuli. The decreased distance
between the nodes of Ranvier will result in nerve stimuli taking longer to get to
the spinal cord. Additionally, the degeneration of myelinated fibers in the spinal
cord will slow impulses as well. Overall, the following anatomical changes will
affect the speed in which the initial impulse translates to a perceived action by
the brain: reduction in conduction velocity of sensory nerves, loss of sensory
axons, reduction in number of Meissner’s corpuscles, reduction in number of
Pacinian corpuscles, decrease in distance between nodes of Ranvier, and
degeneration of some myelinated fibers of the spinal cord.

Changes in Functional Capabilities

Slowed Response To Tactile Stimuli
Slowed Response To Vibratory Stimuli
Decreased Two-point Discrimination
Decreased Kinesthetic Awareness
Changes In Pain Perception
Accelerated Loss Of Audio And Visual Acuity
Postural Instability
Exaggerated Body Sway
Increased Problems With Balance
Wide-based Gait
Growing Difficulty In Recognizing Body Positions In
Space
Worsening Tendency To Drop Items In Hand

Any changes in sensory input have a direct impact upon the functional
capabilities of an elder. Listed below are some common functional changes as
one ages. Having said this, you should keep in mind that different individuals
age differently, and any decrease in physiology, anatomy, or function will be
unique to each individual. However, be cognizant that these are potential threats
to older adults.
slowed response to tactile stimuli
slowed response to vibratory stimuli
decreased two-point discrimination
decreased kinesthetic awareness
changes in pain perception
accelerated loss of audio and visual acuity
postural instability
exaggerated body sway
increased problems with balance
wide-based gait
growing difficulty in recognizing body positions in space
worsening tendency to drop items in hand

In Conclusion…

The purpose of this course was to review skin anatomy as it relates to
occupational and physical therapy practitioners. Our patients, no matter the age,
can present with a variety of diseases or conditions that impact on skin. The
modality that we use to treat our patients will have to transfer through skin.
Applying a hot pack or paraffin treatment can be influenced by the skin’s
condition. The thinner skin found in older adults will require more attention in
order to decrease the possibility of burns. The effectiveness of electrotherapy
modalities can be influenced by the condition of the skin. The dryer the skin, the
greater the impedance and this may result in a more uncomfortable experience
for the patient. Finally, wound healing has a direct correlation with the nature of
the skin and its ability to proceed through the regenerative process. Skin is more
than just a covering for our bodies. Better knowledge of the properties of this
largest organ of the body can help us in our intervention strategies.

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Dellon AL (1997). Somatosensory Testing and Rehabilitation. The
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Calianno C (1999). Skin Care: Keeping the Outside Healthy. Nursing.
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Smith KL and Dean SJ (1998). Tissue Repair of the Epidermis and
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McPhee SD (1995). Functional Problems: Typical Patterns of Hand
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