Appendages
There are two types of structures associated with the skin that are called appendages: the Hair and the Nails. Glands found within the skin and various nerve receptors found within the skin will be examine below.

Notice the small vellus hairs

Hair It is estimated that over 5 million hairs are on the body, with about 100,000 on the scalp (blonds have more, redheads less). Although we have as much as most other mammals, human hair is short and thin, providing very little protection from cold. It provides us with more of a physical protection (from sun, dust, etc) and for sensation. Two types of hair is found: Course, long, pigmented hair such as on our heads is called terminal hair. Covering our bodies, regardless of gender are thousands of small, thin hairs called vellus hair (see image to the right)   Hair Structure Hair is made up of a shaft which protrudes above the skins surface and a root which is embedded in the dermis. Around the root is a complex structure called the hair follicle, made up of epithelial and connective tissues, which is really just a deep extension of the epidermis.

The follicle rests on a dermal papillae, thus receiving nourishment from the blood vessels running through these. The epithelial cells become keratinocytes, producing high quantities of keratin, from which hair is composed. As the cells grow and divide, the older cells get pushed up and away from the blood supply they die. Their highly keratinized remains form the shaft of hair (along with melanocytes who have also dies and "joined for the ride!").

Like skin color, hair color is genetically determined. It's a combination of varying amounts and forms of melanin. Red hair, for instance is a combination of melanin and iron atoms. Dark hair is made of cells producing lots of melanin; blond hair from cells producing very little. As you age, the amount of melanin produced is deminished, thus white hair is produced (or a hybrid of dark and light), thus we 'grey'.

Hair grows in stages dependent in part about the type of hair it is: eyelashes grow for 30 days, then remain inactive for 100 days. Scalp hair grows for 3 years, then rests 1 or 2 years. Hair will grow at a rate of about 2 mm a week. When hair is lost (falls out) it normally means new hair is growing. Many other factors may cause hair loss including a protein deficient diet, disease, drug use (including cancer treatments), old age (after about age 40), and genetic predisposition. Male pattern baldness (on the top of the head) is a genetic condition inwhich male hormones effect the hair follicle, causing to to slow produce less hair, until the hair is so short and thin it is virtually invisable. Rarely is it totally gone.

Each follicle is attached to a strip of smooth muscle called an Arrector Pili muscle, which also attaches to the papillary layer of the dermis. Naturally hair lies close to the skin, but upon stimulation, these muscles pull on the follicle, causing the hair to stand on end. Goose bumps are also caused in this manner. This apears to be one of those "animal characteristics" that remain from our past ancestory. In hairy mammals, raising the hair allowed for pockets of air to form amongst the hair, creating an insulatory effect. With our scant amount of hair, it is likely that we get no benefit from this act.

Nails

Nails are produced by modified keratinocytes and offer assistance with grasping objects. Each nail has three parts: The loose edge, the body and the root. At the base of the nail body is a thickened, light colored region called the lunula. The base of the body, a portion of the lunula, and the nail root are covered by an area of strata corneum called the cuticle. Beneth the lunula is a patch of epidermis (strata corneum) that is the area for nail growth, the matrix. A nail grows about 1 mm a week (about 5 cm per year total growth). Nails do not have a resting period. Toe nails grow at a slower rate. The cells of the matrix grow and keratinze, forming a plate-like structure known as the nail.

Severe damage to the nail matrix can cause the nail to cease growing. The nail's major function is to provide a solid area for the finger tip to push against while one grasps an object. This inturn creates a frictional force on the object, allowing for a 'surer grip'.

Glands
The glands of the skin allow for us to keep oour skin in good condition and also assist our bodies in temperature control. Two major types of glands are found in the skin: Sebaceous glands and Sweat glands.

Sebaceous Glands
These glands are the smallest of skin glands and are found associated with the upper portion of the hair follicle and produce a substance called sebum. Sebum is a white, oily substance with a high lipid content. Sebum helps lubricate the hair shaft, keeping them from becoming brittle and keep the scalp moist, yet waterproof. Reduction of sebum output can cause dry scaly skin. Sebum has a secondary action of protection form some bacteria. These glands are found all over the body, with the notable exception of the palms of the hand, sole of the feet, lips and genitals. After the onset of puberty, sex hormones can cause some glands to become overactive. Dust and grime can become intrapped within a hair follicle and can cause bacteria to thrive within a plugged follicle. This can lead to the production of a pimple.

Sweat Glands

Note the sweat pores of the finger prints enlarged

Sweat glands exist in one of two types: eecrine glands or apocrine glands. Eecrine glands are the most numerous, as many as 3 million found on the body, and are most numerous on the neck, back and forehead, although they are also common on the palms and soles (and absent on lips and genitals). Most sweat from physical exertion or on hot days comes from these glands. The secretions of eecrine glands is primarily water interspersed with some salts and other products. These coiled glands with long ducts opening directly into the skin secrete Na, K, and Cl ions (thus it's salty taste and nature), some nitrogenous waste (urea, ammonia, and uric acid), latci acid, ascorbic acid, antibodies and a small amount of sugar. Up to 1 liter of sweat a day is produced under 'normal' conditions.

The salt residue that is left behind as the water evaporates as well as an enzyme, lysozyme, that is produce both have antibiotic properties reducing bacterial and fungal growth on the skin. As body temperature increases, a signal to the glands is produced that stimulates them to begin excreting sweat. This action can also be created by emotional stress, leading to what we frequently refer to as 'sweaty palms'. Apocrine glands are found within the arm pit, around the gentialia and anus. These glands have their ducts empting into the hair follicle and not directly onto the skin's surface. Apocrine glands secrete an oily (lipid-based) substance upon which the skin bacteria in these regions can feed upon.

Note the microscopic beads of sweat that are continously produced by sweat glands. This from the face, notice the cut beard 'stubble'

Although this substance is initially odorless, the waste products of the bacteria create an odor we frequently refer to as Body odor, or B.O.. Antiperspirants work on the premise of closing these glands. With a decrease in excretions, bacteria don't survive, reducing the odor production.

Apocrine glands become active with the onset of puberty and typicaly active during 'stressful' times. Stimulation by the sex hormones is responsible for activating these glands. It is thought by some that these glands maybe human 'Scent glands', a notion which is contriversal. The role of scent glands in animal behavior, in particular sexual behavior, is well documented. Whether scent has an unconcious effect on human interactions is one for open debate. Activity in women seems to be related to the menstraul cycle, and is is documented that women who spend ample time together have a tendency to synchronize their cycles. Is this related to scent?????

Modified sweat glands within the ear canal produce ear wax. Glands on the inner surface of the eyelids and at the base of eyelashes produce a substance that prevents the lashes from sticking together. Infections in these glands produce what we call a sty.