Hair is a fibrous material which grows from follicles covering the bodies of mammals.http://animaldiversity.ummz.umich.edu/site/topics/mammal_anatomy/hair.html Hair grows from structures in the skin called follicles.http://animaldiversity.ummz.umich.edu/site/topics/mammal_anatomy/hair.html At 22 weeks gestation, the human fetus has developed all the hair follicles in the body, which total about 5 million, 1 million of those forming on the head.http://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair

The average human has about 100,000 hairs on his or her head and sheds 50 to 100 of these on a daily basis.http://www.mayoclinic.com/health/hair-loss/DS00278/DSECTION=causes Typically, hair grows at a rate of about 6 inches every year.http://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair?page=2 Hair gets its color from melanin, a pigment which forms before birth.http://www.loc.gov/rr/scitech/mysteries/grayhair.html

Hair is made up of three layers. The central core of hair, which gives hair its strength, is called the medulla.http://www.opendemocracy.net/arts-hair/article_721.jsp The inner part of hair, which gives hair its shape and holds oils secreted from the sebaceous gland to moisturize hair, is called the cortex.http://www.opendemocracy.net/arts-hair/article_721.jsp The outer layer of hair, the cuticle, is filled with a hard protein called keratin and protects the inner layers from harm.http://www.opendemocracy.net/arts-hair/article_721.jsp

Hair grows in three distinct phases: the anagen, catogen, and telogen.http://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair?page=2 Different strands of hair will be in different phases at any given time.http://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair?page=2

Anagen phase: During this phase, hair is actively growing. For hair on the scalp, this phase can last from two to six years and involves growth of new hair at the follicle which pushes club hair, or hair that has stopped growing, out of the follicle.http://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair?page=2 People with hair that naturally grows longer tend to have a longer anagen phase than those whose hair does not.http://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair?page=2

Catogen phase: This phase marks a transition between the anagen and telogen phase in which hair stops growing and shrinks, attaching itself to the root and becoming club hair. http://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair?page=2

Telogen phase: The telogen phase involves a period of rest for club hairs. http://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair?page=2 Lasting roughly 100 days, this phase accounts for the typical daily hair loss the average human experiences.http://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair?page=2