Scientific Concepts: Hair, Skin, Scalp, Chemistry, and Anatomy
Hair structure and growth
Every hair has three layers: the cuticle (overlapping outer scales that protect and, when damaged, cause dullness and breakage), the cortex (the largest layer - about 90% of the hair - holding strength, elasticity, and melanin), and the medulla (the innermost core, sometimes absent in fine hair). Growth runs in a cycle: anagen (active growth, roughly 2–6 years), catagen (short transition, about 1–2 weeks), and telogen (resting/shedding, roughly 3–5 months). Shedding 50–100 hairs a day is normal.
Chemistry
Hair is mostly keratin, built from the COHNS elements - carbon, oxygen, hydrogen, nitrogen, sulfur. The pH scale runs 0–14; hair sits near 4.5–5.5 and skin's acid mantle near 5.5, so alkaline products swell and roughen the cuticle. The three side bonds are hydrogen (broken by water plus heat), salt (broken by strong acids/alkalis, re-form on drying), and disulfide (broken by thio relaxers and permanent-wave solution).
Anatomy
Name the 8 cranial bones and 14 facial bones, the frontalis, occipitalis, temporalis, and masseter muscles, the trigeminal (chief sensory nerve of the face) and facial (chief motor nerve) nerves, and the superficial temporal, occipital, and facial arteries.
Traps
- Cortex, not cuticle, provides strength and elasticity.
- Hair above the scalp is dead keratinized protein: cutting or shaving never changes growth rate, thickness, or color.
- Each whole pH unit is a tenfold change, not a doubling.
- Porosity (absorption) and elasticity (stretch-and-return) are different tests.
Worked example
A solution at pH 3 versus pH 5: 5 − 3 = 2 steps, and 10² = 100, so pH 3 is 100 times more acidic than pH 5.