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The word "gelid" in its denotative sense represents something frosty and that causes an intense cold. When we analyze this word in the description of the snakes of Medusa's head, we can consider that this word was used to describe the feeling of dread that the snakes caused, because the extreme cold is felt by people who are terrified. Therefore, we can consider that the snakes of Medusa's head were "gelids" because they managed to cause dread and a supernatural fear in those who saw them.