When “ʿayn” is used for a watcher, its salience is clear: observation is primarily associated with the eyes. Merely naming a watcher after any unrelated organ would not convey the person with the same intelligibility.
Part-for-whole transfer requires more than physical membership. The selected part should direct the hearer to the whole in a meaningful way.
Two questions therefore matter:
1. Is the named thing genuinely a part of the intended whole?
2. Does that part provide a sufficient route to identifying the whole?
This distinction separates a justified figure from arbitrary naming.