Relating to or connecting the clavicle (collarbone) and the mastoid process (the bony bump behind the ear).
From Greek 'kleido-' (clavicle) plus 'mastoid' (breast-shaped, from 'mastos' meaning breast), which describes the mastoid process's bump-like appearance.
The mastoid process is called 'breast-shaped' because ancient anatomists thought it resembled a breast—it's one of those embarrassing etymological moments in medicine where description meant describing it the way it looks, leading to some anatomically... creative naming conventions.
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