In Jane Eyre, what are chilblains?

A chilblain is a painful swelling or sore caused by exposure to cold, especially on the fingers, toes, or ears. In the novel Jane Eyre, Jane describes how she and other students suffered through the winters at Lowood, a charity school for orphan girls:

Our clothing was insufficient to protect us from the severe cold: we had no boots, the snow got into our shoes and melted there: our ungloved hands became numbed and covered with chilblains, as were our feet . . .