A refiner comprising a housing having a refining chamber providing a flow path for particulate material to be refined while travelling between an upstream inlet and a downstream outlet. A refining assembly in the chamber has a series of relatively rotatably cooperative axially confronting annular refining surfaces located on partially internested refining disks and defining radially extending refining zones therebetween and with radially opposite ends of the zones closed. Passages defined by and between the internested portions of the disks connect the adjacent refining zones in a manner to cause the particulate material such as paper making stock to pass successively between the radially outer ends of the zones and the radially inner ends of the zones.