make your blood run cold horrify you.

☞ The previous three phrases all come from the medieval physiological scheme of the four humours in the human body (melancholy, phlegm, blood, and choler). Under this scheme blood was the hot, moist element, so the effect of horror or fear in making the blood run cold or curdling (solidifying) it was to make it unable to fulfil its proper function of supplying the body with vital heat or energy. The blood boiling was a supposedly dangerous overreaction to strong emotion.