Anatomy, Physiology, and Histology of the Hair and Skin
The hair
Each hair grows from a follicle; the bulb at its base sits over the papilla, which supplies blood. The sebaceous gland opens into the follicle. The shaft has three layers: the cuticle (protective overlapping scales), the cortex (the bulk, containing melanin and the disulfide bonds that color and texture services alter), and the medulla (a central core that may be absent, especially in fine hair). Hair is mostly keratin. Growth cycles are anagen (active), catagen (transition), and telogen (resting and shedding).
The skin
The epidermis has five strata; the stratum basale produces new cells and the stratum corneum is the protective outer layer. The dermis holds collagen, elastin, blood vessels, nerves, and glands. The subcutaneous layer stores fat. Appendages include hair follicles, sebaceous glands, eccrine sweat glands, and nails.
Head, neck, and hands
Bones: frontal, parietal, temporal, occipital, zygomatic, mandible. Muscles: frontalis, occipitalis, temporalis, masseter, sternocleidomastoid. Nerves: the fifth cranial (trigeminal) and seventh cranial (facial). Arteries: temporal, occipital, and the superficial temporal branch.
Worked example
Why does permanent color change hair permanently? The alkaline developer swells the cuticle and lets dye intermediates enter the cortex, where they combine into larger molecules that are trapped inside. They cannot be shampooed out.
Traps
Thinking the cuticle absorbs color directly. Confusing the sebaceous gland's position with the sweat gland's. Assuming a medulla is always present. Mixing up arteries and veins during scalp massage.