Human or Beast? The Medullary Index Mystery

Welcome, Investigators!

A single hair can settle a huge question before you know anything else: did a person leave it, or did an animal? Get that wrong and the whole case wanders off course. Today you'll measure the hollow core running down a hair and let one ratio tell you human from beast. Sharpen your focus. Follow the evidence!

The Case

A struggle. A victim found with a single hair clutched in one hand — pulled, by the look of the root, from whoever or whatever they grabbed. Before the lab spends weeks on DNA, the first question is the cheapest and fastest one to answer: is this hair even human? The scene had a lot of foot traffic:

• The victim's own dog, a shedding shepherd mix, was in the room.
• A wool blanket was tangled at the scene.
• A suspect with medium-brown head hair was reported fleeing.

Your job: mount reference hairs from human, dog, cat, and wool, measure each one's medulla and shaft under the microscope, compute the medullary index, and classify the mystery hair — human or non-human — using the numbers, not a guess.

Learning Objectives

By the end of this investigation you will be able to:

1. Prepare a wet-mount slide of a hair sample for compound microscopy.
2. Measure the medulla diameter and shaft diameter of a hair using a calibrated scale.
3. Calculate the medullary index as the ratio of medulla to shaft diameter.
4. Classify a hair as human or non-human by comparing its medullary index to reference ranges.

Quick Facts

Lab type	🔀 Combination (microscope bench + MicroSim verdict)
Group size	2–3 investigators
Time	45–55 minutes
Cost	≈ $15 per group (consumables)
Ties to	Ch 4 — Hair Medulla Structure, Medullary Index Calculation, Human vs Non-Human Hair, Hair Scale Patterns

Materials

Per group (≈ $15):

• Reference hairs: a human head hair (volunteer), dog fur, cat fur, and a strand of wool yarn
• 1 "mystery" hair (provided sealed by your teacher)
• Blank glass slides + coverslips
• Clear nail polish, glycerin, or water as a mountant
• Tweezers / forceps
• Ocular micrometer or a printed scale bar / calibrated reticle
• Lab notebook and calculator
• Shared: the AFIS-style Medullary Index Calculator MicroSim (below)
• Assumes access to: a classroom compound microscope (100×–400×)

Safety & Fair-Test Rules

• Glass slides and coverslips chip and cut — handle by the edges and tell your teacher about any breakage. Goggles on while mounting.
• If you use nail polish as a mountant, work in a ventilated area; the fumes are strong.
• Measure the same way every time — always the widest part of the medulla and the widest part of the shaft at the same spot. Inconsistent measuring points are the number-one source of a wrong index.

Background: The Hollow Core That Tells the Tale

Look down the length of a hair under magnification and you'll see three layers. The outer cuticle is a shell of overlapping scales. Beneath it, the cortex carries the pigment that gives hair its color. And running down the center is the medulla — a channel of cells that may be a solid rod, a beaded string, a wide hollow tube, or absent entirely. That central channel is the key to this whole lab.

Forensic examiners quantify the medulla with the medullary index: the diameter of the medulla divided by the diameter of the whole hair shaft.

\[\text{Medullary Index} = \frac{\text{medulla diameter}}{\text{shaft diameter}}\]

The rule of thumb is simple and powerful. In human hair the medulla is thin, fragmentary, or missing, so the index is less than about 0.33 — the medulla takes up under a third of the shaft. In most animal hair the medulla is broad, so the index is 0.5 or greater — half the shaft or more. A wide, patterned core screams animal; a narrow or absent one points to human.

A quick, honest caveat: hair is class evidence. The medullary index can tell you human versus animal, and scale patterns can even narrow the species, but a hair alone rarely identifies one individual person. That's a job for the DNA in the root. The 2009 NAS report specifically criticized overstated microscopic hair "matches," so we measure carefully and claim only what the number supports. Practice the calculation before you touch a slide.

Explore: Medullary Index Calculator

Medullary Index Calculator Interactive MicroSim

Type: microsim
sim-id: medullary-index-calculator
Library: p5.js
Status: Specified

Learning Objective: Calculate the medullary index from medulla and shaft measurements and classify a hair as human or non-human (Bloom Level 3 — Apply).

Enter a medulla width and a shaft width and watch the index update, then see where it lands against the human and animal reference ranges. Notice the gray zone between about 0.33 and 0.5 — a value in there is a warning to remeasure and to reach for scale-pattern evidence before you commit.

Procedure

Part 1 — Mount the reference hairs.

1. Lay one reference hair (start with the known human head hair) flat on a clean slide. Add one drop of mountant and lower a coverslip at an angle to avoid trapping bubbles.
2. Repeat for the dog, cat, and wool references, one slide each. Label every slide.
3. Mount the sealed mystery hair on its own labeled slide the same way.

Part 2 — Measure under the microscope.

1. Focus on a reference hair at medium power. Using your ocular micrometer or scale bar, measure the shaft diameter (full width of the hair) at a clear spot.
2. At the same spot, measure the medulla diameter (width of the central channel). If no medulla is visible, record it as 0.
3. Record both numbers for every reference hair and for the mystery hair. Note the medulla's appearance too — solid, beaded, ladder-like, or absent.

Part 3 — Calculate and classify.

1. For each hair, compute the medullary index = medulla ÷ shaft. Check your value in the MicroSim.
2. Compare each index to the reference ranges: < 0.33 → human, ≥ 0.5 → animal, 0.33–0.5 → inconclusive, remeasure and use scale pattern.
3. Classify the mystery hair and decide which scene source it is consistent with — the suspect, the dog, or the wool blanket.

Data Collection

Hair sample	Shaft diameter (µm)	Medulla diameter (µm)	Medullary index	Medulla appearance	Classification (human / animal)
Human (known)					human
Dog
Cat
Wool
Mystery hair

Analysis Questions

1. What is the medullary index of the mystery hair, and how does it classify — human or animal? Cite the number and the range it falls in.
2. Which scene source (suspect, dog, or wool) is the mystery hair consistent with? Which sources can you exclude?
3. Compare the medulla appearance of your human hair to your animal hairs. Describe two visible differences beyond the index number itself.
4. A hair gives a medullary index of 0.40 — right in the gray zone. What two things should you do before reporting a classification?
5. Hair is class evidence. The 2009 NAS report criticized overstated hair "matches." Explain why your report can say the hair is consistent with a source but should not say it came from one specific individual.

Deliverable

Turn in a one-page Hair Analysis Report that gives the mystery hair's medullary index, its human/animal classification, and which scene source it is consistent with (and which it excludes). Include your full data table and a note on any measurement you'd repeat to be sure.

Investigator Tip

Measure the medulla and the shaft at the exact same point along the hair, and use the widest clear view of each. A medulla that beads and pinches will hand you three different indices in three different spots — pick a representative location and be consistent, or your ratio is fiction.

Extension Challenge: Read the Scales

The medullary index isn't the only clue. Make a scale cast by pressing a hair into a thin layer of clear nail polish on a slide, letting it dry, and peeling the hair away to leave a scale imprint. Human scales look flattened and irregular; many animals show crown-like (coronal) or spiny (petal) scales. Cast two of your samples and describe how the scale pattern confirms your index-based classification.

Teacher Notes

Setup, timing, and grading (click to expand)

• Prep: Collect reference hairs ahead of time — a volunteer head hair, brushed dog and cat fur, and a single-ply wool strand. Choose a mystery hair that gives a clear, non-borderline index so groups get a decisive result their first time out.
• Calibration: If you have ocular micrometers, calibrate them against a stage micrometer before class. If not, a printed scale bar photographed at known magnification works for relative ratios (the index is unit-free, so consistent units are all you need).
• Differentiation: For a shorter version, pre-mount the reference slides and have groups measure only. For a challenge, add a second mystery hair that lands in the 0.33–0.5 gray zone to force a remeasure-and-scale-pattern decision.
• Assessment focus: Reward consistent measuring points, a correctly computed index, and cautious class-evidence language — consistent with, not identical to. Flag any report that claims a single individual from a hair.

Case Closed — For Now

One hair, one ratio, and you split human from beast before the DNA lab even opened the envelope. That's the power of a fast, cheap screening test done carefully. You measured twice and claimed only what the number allowed — exactly what a courtroom expects. Follow the evidence!