The spleen is an organ in the upper left abdomen, which filters blood, disposes of worn-out red blood cells, and provides a 'reserve supply' of blood. It contains both red tissue, and white lymphatic tissue. The major (encapsulated) lymphatic organs are the lymph nodes, thymus and spleen.
Place the brined corned beef in a large pot or Dutch oven along with the liquid and spices that accompanied it in the package. Pour in enough water to cover the beef, then bring the water to a boil on the stovetop. Reduce the heat to a simmer, and cover the pot. A three-pound corned beef could take three hours or more to become perfectly tender. Check the meat occasionally, adding more water if necessary. The beef is ready when it pulls apart easily.
Spare ribs and pork shoulder are both cooked in an oven at the same temperature(~225F) for me. The difference is time. A pork shoulder takes at least 8 hours but usually around 12 hours to give good pulled pork. Ribs are closer to 3-4 hours for the meat to get tender but not so tender that they fall off the bone. I have never cooked country style ribs but from your description it seems like it is somewhere between these two cuts of meat. I would cook it at the exact same temperature but start to check after 3 hours.