Abstract: An improved circular anastomosis surgical stapling instrument for joining hollow tubular organs, has a body having a staple-carrying assembly at its distal end, a centered longitudinally extensible and retractable main shaft centered in the body, and an anvil opposed to the staple-carrying assembly, and is characterized by a separate shaft segment frictionably connectible with the end of the main shaft, the shaft segment having a free end and an end carrying the anvil. The main shaft has a hollow free end and the free end of the shaft segment is releasably receivable in the hollow end of the main shaft. In a preferred form the end of the main shaft is provided with a plurality of several radially-extending arms positioned to overlie the main shaft but having spring hinges biasing them radially outwardly away from the main shaft. The instrument further includes a second shaft segment releasably receivable in the hollow of the main shaft which second shaft segment has a concial pointed unit at its distal end.