They call it a slave pen, but it’s really a cage. the wooden structure, 20 by 30 feet and two stories high, imprisoned slaves who had been bought by a Kentucky dealer until he could dispose of them. Inside, men were chained in place, while women could move about, cooking in a fireplace and doing their best to deal with the tight quarters, tiny windows, and complete lack of sanitation. The pen, later converted to a barn, was bought and restored by the National Underground Railroad Freedom Center , in Cincinnati, and will form the centerpiece of that museum when it opens on August 23. Other exhibits will tell the story of the secret network that helped 50,000 slaves escape to freedom and of the people who risked their lives to keep it running. In keeping with the second part of its name, the museum will also examine issues in freedom and human rights worldwide, past and present, and encourage visitors to take action in their own communities. For more information, see the center’s Web site, www.freedomcenter.org.