It's about as old and dilapidated as a bus can be. It's been used mostly as a storage shed for the past 30 years. But an anonymous cultural institution just paid $492,000 for it at auction and will doubtless spend another significant sum fixing it up. It's the bus that sparked the end of segregation in the South. Actually, Rosa Parks, who was on the bus in Montgomery, Alab., on Dec. 5, 1955, did the sparking. As she rode home from work, a white man demanded that she give up her seat, as the law required. She refused, the driver had her arrested and she was fined $14. But that started a 381-day bus boycott in which Martin Luther King Jr. emerged as a civil rights leader and saw his house get blown up. The Supreme Court ultimately ruled that segregation on city bus systems was unconstitutional and American apartheid began to come apart. Parks is now 88 and in ill health, but she stated her reasoning in 1956: "Well, in the first place, I had been working all day on the job. I was quite tired after spending a full day working. I handle and work on clothing that white people wear. That didn't come in my mind but this is what I wanted to know: When and how would we ever determine our rights as human beings?. . . It just happened that the driver made a demand and I just didn't feel like obeying his demand."