Syringe with cannula-protecting sheath and sealing center rod

In a syringe of the type that is prefilled with a substance for both storing the substance within the syringe and later ejecting the substance from the syringe, the improvement comprising:

• a syringe barrel having an axial length with a cannula collar at one end of the barrel length and an interior chamber within the barrel;
• a hollow cannula having axially opposite proximal and distal ends, the proximal end being secured to the syringe barrel collar with the cannula extending axially from the collar to its distal end;
• a bore having a length extending axially through the cannula and the syringe barrel collar between the cannula distal end and the syringe barrel interior chamber, the bore having an interior surface where at least a segment of the length of the bore changes in diameter from a first interior diameter of the bore to a second interior diameter of the bore, less than the first interior diameter, as the bore extends in the direction from the cannula distal end to the barrel interior chamber through the bore segment;
• and,
• a center rod having a diameter smaller than the first interior diameter and larger than the second interior diameter, removably inserted axially into the bore at the cannula distal end and seating in a sealing engagement with said segment of the interior bore.