Hair, nails and glands all start as epidermis that grows down into the dermis. Grow a hair through its whole cycle, grow out a nail and read the record it keeps, compare hairs like a forensic scientist, then find the three glands of the skin and run a sweat-print test.
Follow-along slideshow · 23 slides that walk through the worksheet, question by question
Printable student worksheet (PDF) · two pages, Day 1 on the front, Day 2 on the back
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Teacher guide: NYSSLS, SEP and CCC alignment (PDF)
This is a model, so every run gives the same numbers. The drawings are not to scale. Growth values are rounded typical adult values: scalp hair grows about 1 cm a month (about 0.35 mm a day) with a growing phase of 2 to 7 years (3 years here); eyebrow hair grows about half as fast with a growing phase of a few months (2 here); the stopping phase lasts 2 to 3 weeks and the resting phase about 3 months. Fingernails grow about 3.5 mm a month and big toenails about 1.6 mm a month (Yaemsiri and others, 2010); real rates vary with age, season and health. The nail line here starts at the cuticle; a real one forms a few millimetres further back, under the skin. Hair widths and medulla widths are typical values; the rule that a medulla wider than about a third of the hair points to an animal comes from forensic hair guides, and microscopy alone can only say a hair is consistent with a source, not prove whose it is. Gland densities are rounded averages of published counts (palm about 520 sweat glands per cm², back about 100); which glands switch on for each trigger, and how many, are model values that keep the real pattern. Anatomy & Physiology, Hole's Essentials 6.3: Accessory Structures of the Skin.