Harrasement /discrimination angry whats next

i havge been working at my job for three years now and i have had this specific boss now for almost one year. at first he was really nice and fair but over the year he has changed very much. in september of 2008 i was called into his room as well as my immediate boss and another supervisor. the other supervisor and myself had gotten into a discreet disagreement regarding our patient flow the day before. When the door closed to my bosses office he began yelling at me telling me he would make my life a livng hell to the point where i would quit. he also told me to shut the hell up and sit down once i would try to defend myself and speak on my behalf. He belittled me and told me that who would corporate believe me or a doctor(him)? while the other people where in the room no one stood up for me. then once he was finished he told me to get the hell out of his room. i continued to work that day. the next morning he apologized for what he said. there have been other times that he has said rude remarks to me either regarding my race or calling me a bitch! i dont respond back i just chuckle a bit because if i were to get angry or say something back i will be fired. i have reported it to higher authorities but i have gotten no where. some of the girls at work dislike me for whatever reasons but they have filled his head with malicious things and he says what he pleases and i am so angry for being treated this way i do my job everyday and my immediate boss never has had a problem with me.please help i am one step away from going off!!