1. The behaviour
audited ground
Many species orient by the geomagnetic field; the compass can be disrupted by weak oscillating fields and depends on light. That much is behavioural experiment, repeatedly confirmed.
Chapter 4e · scaffold · structure only · the author writes the prose
The Magnetic Sense, Held Open
Migratory birds orient by the Earth's magnetic field — the behaviour is well established. How they do it is not. The leading hypothesis involves quantum effects in a light-sensitive protein in the eye. This chapter exists to show the book's discipline: a beautiful idea, labelled honestly as unsettled.
2. The hypothesis
held open — unsettled
The radical-pair hypothesis: light striking cryptochrome proteins in the retina creates pairs of radicals whose chemistry is sensitive to the direction of the Earth's field. It would be biology using quantum coherence at body temperature — exactly the kind of thing this book wants to be true, which is why it needs the tighter grip.
3. Why it stays open
held open — unsettled
The cryptochrome chemistry is real; the step from molecule to a working compass in a living bird has not been demonstrated. The chapter can use this as the standing example of an idea carried honestly: exciting, plausible, unproven.
Your framing: why this chapter is in the book at all — the demonstration of how the book treats a claim it loves.
Your leap
Your leap: if a sense for the planet's field is real, what does that do to the reader's picture of what a body is in contact with?