Hair refers to the hair that grows on the head. Hair is not an organ, so it does not contain nerves and blood vessels, but cells. Hair is mainly used to protect the head. Soft and fluffy hair is elastic, which can resist light collision and help head sweat evaporate.

On average, each person has about 654.38+ million hairs. Of all the hairs, the length of hair is the longest, especially for women with long hair. Some can grow to 95 ~ 100 cm, or even 150cm, but generally it will not exceed 200cm. Hair color is generally determined by genes, such as black, golden yellow, brown, brown, gray and red.

When people get old, their hair usually turns silvery white. Due to different races and regions, hair is black, golden, reddish brown, reddish brown, yellow, gray, and even green and red. Scientific research has proved that the color of hair is related to the difference of metal elements contained in hair.

Black hair contains equal amounts of copper, iron and melanin. When the content of nickel increases, it will turn gray. Blonde hair contains titanium, reddish-brown hair contains molybdenum, reddish-brown hair contains cobalt besides copper and iron, and green hair contains too much copper. In some African countries, some children have red hair, which is caused by the serious shortage of protein.

Hair follicle is the part of hair root in dermis, which consists of inner hair root sheath, outer hair root sheath and hair ball. Inner hair root sheath is the sheath layer directly adjacent to hair in the later stage of hair development. The inner hair root sheath is a hard thick-walled keratinized tube, which determines the shape of the cross section when the hair grows.

The lower part of the inner hair sheath has three layers: Huxley sheath, Henler sheath and the surface layer of the inner hair root sheath. Before hair keratinization, the inner hair root sheath grows with hair, and its source is the cells propagated at the bottom of hair follicle. Near the epidermis, the inner hair root sheath is separated from the epidermis and hair follicles.

The function of sebaceous glands is to secrete sebum and squeeze it out through the sebum tube. When the hair passes through the sebum tube, it will take away the sebum squeezed out of the sebum tube. Sebum provides natural protection for hair and gives it luster and waterproof performance.

Hair muscle is a small muscle organ connected with epidermis, which depends on the external physiological environment. Hairy muscles can stretch or contract. The drop in temperature or the action of adrenal hormones can pull the hair follicle to a higher position and make the hair stand up.