What's the difference between 下巴 and 頷? chin seems to be translated as both 下巴 and 頷. Is there absolutely no difference? Is one more common than the other? I find it strange that there are two translations for so many words in Chinese, especially for specific words like chin that have little ambiguity.

When I look up pictures for the word 頷, I see more medical pictures, for 下巴, more regular pictures. Is 頷 something more concrete than chin? My dictionary says just "chin".
For modern Chinese, 下巴 is more commonly used in the daily life for the meaning of jaw or chin. Whereas 頜 is often used in medical words, like 下頜骨.

In Ancient Chinese, 頜 is more common.


  頷、頤,頜也。南楚謂之頷,秦晉謂之頜。頤,其通語也。――《方言》


ref: http://baike.baidu.com/view/269020.htm