Anatomy & Physiology
A hair is an appendage of the skin. It grows out of, and is part of the skin, and is made up of dead skin cells containing keratin. The palms of the hands, soles of the feet, lips and some parts of the sex organs are the only parts of the skin which are not covered in hair.
The hair has a number of different functions, including insulation, and protection from bumps and the sun. The eyelashes help prevent foreign particles entering the eyes, whilst nostril hair does the same for the nose. Body hair also provides a sensory function and helps to secrete sebum on the surface of the skin.
There are three types of cells in the hair, which form different layers. The medulla is the central layer and contains soft keratin. This layer only exists in coarser hair, not thinner hair.
The pigment, along with hard keratin is found in the cortex, the thickest layer of the hair. This layer is made up of dense, elongated cells. It is these cells which contain the pigment and hard keratin. The pigment is what gives the hair its colour, and once this stops being produced, the hair becomes white. Tinting products colour the melanin in the hair, which is why grey or white hair is more difficult and time consuming to treat.
The outer layer of the hair is the cuticle. The cells contained within the layer are thin and flat, and contain hard keratin.
The hair is made up of a root, the part of the hair within the follicle, the bulb, which is the base of the root, and the shaft, which can be seen above the surface of the skin.
The hair grows out of the follicle, which is a continuation of the epidermis. The movement of the hair is controlled by the arrector pili muscle, which is attached to the base of the follicle.
The muscle contracts, and pulls the follicle and hair upright.
The sebaceous gland produces sebum, which is secreted into the follicle. The purpose of sebum is to soften the hair and skin and protect against infection.
The growth of the hair comes from the dermal papilla. This has a good blood supply, and is a separate organ which serves the follicle.
The matrix is the lower part of the bulb and divides the cells from which the hair is formed.
The hair follicle is made up of the inner epithelial root sheath, which is covered with cuticle cells and anchor the hair, the outer epithelial root sheath which forms the follicle wall and the connective-tissue sheath which provides a sensory and blood supply.
As with most functions of the body, the growth of the hair is part of a cycle. The first part of the cycle is the anagen stage, where the hair actively grows. The follicle reforms and a new hair begins to grow from the matrix.
The hair separates from the papilla in the catagen stage. It is carried by the movement of the inner sheath to the sebaceous gland where it stays until it falls out. The telogen stage is the resting stage.
There are a number of patterns of hair growth according to a person’s age, sex or race.
Congenital hair growth is a pattern of growth which is genetic, and that the person is born with.
Topical hair growth is caused by sustained irritation to an area, or by temporary methods of hair removal as the blood supply is stimulated, creating greater hair growth.
Systemic hair growth is brought about by hormonal changes such as puberty, pregnancy or menopause.
Hypertrichosis is an abnormal growth of excess hair, whilst hirsutism describes a pattern which is abnormal for a person’s sex. Superfluous hair is normal at certain points of a female’s life, such as puberty.
There are three main types of hair: lanugo, vellus and terminal.
Lanugo hairs are found on the body prior to birth and are fine with no medulla and are often unpigmented.
Downy body and facial hair is vellus which usually has no pigment with no medulla or fully formed bulb.
Terminal hairs are longer and coarser and carry pigment. Their appearance varies and their follicles are deeply set in the dermis with well defined bulbs. This is the hair that usually makes up eyebrows, eyelashes, the scalp, pubic and underarm areas.
Hair does not grow uniformly and can be affected by factors such as the time of day, weather and hormonal changes. Age, diet and stress can also affect this, whilst darker hair grows more quickly than blonde hair. It is possible to have a hereditary pattern of hair growth, and race can also different patterns.
The eyes are the sense organs which enable us to see.
The eyeball itself is the organ which provides sight, however, the eye also consists of the eyelids, eyebrows, eyelashes and eye muscles which help to protect the eye. They also contain lacrimal glands which release tears. These tears help to lubricate the eyeball, whilst the conjunctiva membrane helps to protect it.
The eyeball is made up of a protective wall and a larger inner space that is divided into cavities. The wall of the eyeball is made up of an outer fibrous tunic layer which is the outermost covering.
This consists of the cornea at the front - which covers the iris and help focus light and the sclera at the back, which gives the eyeball its shape. The next layer is the vascular tunic layer which is fronted by the coloured iris. This iris is suspended between the cornea and the lens, with a central hole for the light-regulating pupil.
The ciliary body which surrounds the iris contains aqueous humour and muscles to alter the shape of the lens when viewing different distances. The ciliary body then becomes the choroid, which lines the internal surface of the sclera and provides nutrients to the retina.
The inner layer of the eyeball is the nervous tunic layer and has a non-visual pigmented portion and a neural portion. The pigmented portion contains melanin and absorbs stray light rays, preventing scattering and ensuring a clear image.
The neural portion has three layers of neurones that process what it is we are seeing. These layers contain photoreceptors made up of rods which only respond to shades of grey, and cones which respond to colour. Information is passed from here to the ganglion cells, the axons of which extend into the optic nerve.
Inside the eyeball, the lens can be found. This is behind the iris and the pupil, and it is responsible for focusing the image. The anterior cavity is found in front of the lens and is filled with aqueous humour, which nourishes the lens and cornea. The posterior cavity lies behind the lens and contains the vitreous body, which produces intraocular pressure and helps keep the retina pressed against the choroid.
Refraction takes place as light passes through the cornea, aqueous humour, lens and vitreous humour. This is because each element has a different density, and so the rays will bend and their speed becomes affected. This reverses the image when it reaches the retina, and the image is transported through nerve impulses by the optic nerve to the visual cortex of the brain. This is the point at which they are interpreted into visual images.
A thickening of the cornea can occur as a result of swelling behind the cornea, usually caused by the eyes being closed for long periods. Once the eyes re-open the cornea reduces its thickness as the swelling reduces, restoring normal vision.
The Meibomian gland is part of the sebaceous glands found on the eyelash line. This can be over-stimulated due to an incorrect choice of lash length or thickness or the wearing of the single lash extensions for long periods.