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Medullary Index Calculator

Run the Medullary Index Calculator Fullscreen

About This MicroSim

A hair shaft has an outer layer called the cortex and a central canal called the medulla. The medullary index (MI) is a simple ratio:

MI = medulla diameter ÷ overall shaft diameter

That one number is a powerful first screen in trace-evidence work. Human hair usually has a thin, fragmented, or absent medulla, so its MI is small (≤ 0.33). Animal hair tends to have a wide medulla that fills much of the shaft, giving a large MI (≥ 0.50). Values in between are ambiguous and need other features to decide.

This MicroSim lets you measure and calculate: drag the two diameters and watch the cross-section, the MI value, and the human/non-human verdict update live.

How to Use It

  1. Drag the Medulla slider to set the medulla diameter (µm).
  2. Drag the Shaft slider to set the overall shaft diameter (µm).
  3. Watch the Hair Cross-Section redraw — the dark inner circle is the medulla, the lighter ring is the cortex, and the arrows show each diameter.
  4. Read the Calculate the Index panel: the formula, the computed MI to two decimals, and a color-coded badge:
  5. Green — Human (MI ≤ 0.33)
  6. Orange — Ambiguous (0.33 < MI < 0.50)
  7. Red — Non-human likely (MI ≥ 0.50)
  8. Press New Sample to load a preset diameter pair and classify it.
  9. Compare your value to the species reference row (human, dog, cat, rodent).

What You Can Learn

  • Calculate the medullary index from two measurements.
  • Apply the threshold rule (0.33 and 0.50) to classify a hair as human, ambiguous, or non-human.
  • Explain why a wide medulla pushes a hair toward an animal classification, and why a borderline MI is not enough on its own.

You can embed this MicroSim on your own web page with this iframe:

<iframe src="https://dmccreary.github.io/forensic-science/sims/medullary-index-calculator/main.html"
        width="100%" height="502" scrolling="no"></iframe>

Lesson Plan

Audience: High-school forensic science (grades 9–12) Time: 10–15 minutes Bloom level: Apply (L3) — calculate.

Worked example. Set the medulla to about 18 µm and the shaft to about 90 µm. The MI is 0.20 — green, Human. Now widen the medulla to about 60 µm on a 95 µm shaft: the MI jumps to 0.63 — red, Non-human likely. The shaft barely changed; it was the medulla that drove the verdict.

Guided questions:

  • What MI value sits right on the human/ambiguous boundary? On the non-human one?
  • If two hairs have the same shaft diameter, which one is more likely animal — the one with the wider or narrower medulla?
  • Why might a forensic examiner refuse to call a hair "human" on MI alone when the index lands in the ambiguous band?

Extension. Look at the species reference row. A rodent's MI is near 0.90. Sketch what that cross-section looks like compared to a human's, and explain how a microscopist could tell them apart at a glance.

References

Specification

This MicroSim was generated from a specification in Chapter 4: Hair, Fiber, and Trace Evidence Analysis.

Design note: the cross-section is a schematic drawn to a fixed scale (SCALE_MAX = 120 µm); it illustrates the medulla-to-shaft ratio, not a photomicrograph of a real hair. The interpretation thresholds (0.33 and 0.50) and the species reference values are typical teaching figures — real casework uses ranges and additional microscopic features, so the MI is a screening tool, not a sole identifier.