The hair is a kind of fibrous tissue grown out of the scalp, which was formed by the arrangement of hardened and keratinized cells of regeneration. The hair can be grouped into four parts of dermal papilla, hair follicle, hair root, hair shaft from down to up.

　　Hair root is below the scalp, and protected by hair follicle. The hair follicle is the part of hair root in the dermal layer, which is a fistulous structure and a very thin tract. The hair follicle was composed of depressed cuticle of skin, and penetrated into dermis. Hair root contains inner root sheath, outer root sheath and hair bulb. The inner root sheath is directly connected with hair at the end of the hair growth phase, which is hard, straight, thick keratinized duct and determine the shape of section when the hair is grown. There are three layers below the inner root sheath: Huxley layer, Henle’s layer, and the cuticle of inner root sheath. Prior to the keratinization of hair, the inner root sheath is grown with hair, which is the result of accumulated cells grown from the bottom layer of hair follicle. The inner root sheath will be separated with cuticle and hair follicle at the place near to cuticle. Generally speaking, there is a hair grown out of a hair follicle.

　　Hair papilla was composed of dermal tissue containing a mass of capillary vessels at the hollow bottom of hair follicle, which is connected with capillary vessels and never endings. Hair papilla and connective tissue sheath can provide nutrients and oxygen for the hair growth. If the hair papilla is destroyed or retrogressed, the hair will stop growth and gradually shed off. At the base of hair follicle, the epidermal cells are continuously divided and differentiated, which can form different composition due to the different way of differentiation such as cortex, epidermis and medulla, the outermost layer of cells can form inner root sheath. During this stage, the cells are soft and not keratinized.

　　There are lots of divided cell nodes on surface of hair papilla, which are called hair bulb and is the origin of hair growth. There are chromatophores produced at the upper part of bulb, which decide the color of hair. There are melanoblasts on the hair papilla of the Asians, so their hairs are black.

　　There is a sebaceous gland, which can penetrate into the dermal layer together with hair follicles. The main function of sebaceous gland is to secrete sebum, the sebum can be squeezed by the sebum duct. When the hair grows through the sebum duct, hair can take the sebum. The sebum can provide hair with natural protection, and moisturize hair, and give the hair luster and water-proof performance. The hair quality can be decided by the quantity of sebum secreted by sebaceous gland, i.e. the neutral, oily or dry hair can be classified.

　　Hair shaft can be divide into three layers, i.e. cuticle, cortex, and medulla. The cuticle is composed of a layer of translucent and scaling inter-laying cells and function on protecting hair. The cortex layer is the most important part of hair, and aligned with long and flat cells at the vertical direction. It contains fibrin and control the moisture of hair. The natural chromotophore includes black, brown, red and yellow. Besides, cortex layer also decides the tenacity, elasticity, plasticity, and intensity. Medulla layer is the core of hair, and composed of transparent, polygonal and keratinized cells, there are pigment existing in the medulla.

　　Arrector Pili Muscle is very small muscle organ connecting with epidermis, it can extend or contract. When the temperature is lowered or under the function of adrenalin, it can pull upward the hair follicle to higher position and erect the hair.