Medical Definition of nuchal line
: any of several ridges on the outside of the skull: asa: one on each side that extends laterally in a curve from the external occipital protuberance to the mastoid process of the temporal bone—called also superior nuchal lineb: occipital crest ac: one on each side that extends laterally from the middle of the external occipital crest below and roughly parallel to the superior nuchal line—called also inferior nuchal line
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