One pleasure of revisiting the early days of any classic TV series is seeing how it began in order to appreciate how it evolved. What's striking about ER is that it came out of the gate almost fully formed, steady-camming along at that now-familiar breathless pace, careening in and out of life-and-death crises while catching glimpses of the often-interwoven personal lives of its doctors and nurses. Nine years later, not much has changed -- except the faces. And while the show's many comings and goings have kept the formula fresh, there's been nothing quite like that original cast -- especially George Clooney and Julianna Margulies -- as well as the long-gone Gloria Reuben, CCH Pounder, and William H. Macy. Rediscovering what they brought to the mix is the real pleasure of this boxed set. It features several making-of docs and commentaries but really only one essential extra: a glossary of ''ER'''s famous medical-speak. Finally, we find out exactly what ''blood gas'' is.