Syringe

1. A syringe comprising:

(a) a plunger including a plunger head and a shaft;

(b) a barrel;

(c) a restrictor mechanism located between the shaft and the barrel and including (i) a first part having an outer member engaged with the barrel and oriented so as to limit movement of the first part with respect to the barrel in a first direction, and (ii) a second part;

(d) the restrictor mechanism and/or shaft carrying a formation for limiting movement of the restrictor mechanism with respect to the plunger in said first direction;

(e) the restrictor mechanism being provided with a further member oriented or arranged so as to restrict movement of the restrictor mechanism with respect to one of the barrel and plunger shaft in a second direction opposed to said first direction; and

(f) a latch member for preventing the further member from restricting movement of the restrictor mechanism with respect to one of the barrel and plunger shaft in said second direction until a predetermined point in an operation cycle of the syringe;

wherein the first part and second part are moveable relative to each other over a limited range of movement to permit unlimited repeated movement of said plunger relative to the barrel in said first and second directions over said limited range.

2. A syringe as claimed in claim 1 wherein the said predetermined point in the operation cycle is an initial movement of the plunger in a distal direction.

3. A syringe as claimed in claim 1 wherein the barrel has a cylindrical interior surface substantially free of discontinuities between proximal and distal ends of a full range of usable movement of a plunger head in the barrel.

4. A syringe as claimed in claim 1 wherein the said formation carried by the restrictor mechanism and/or shaft for limiting movement in the first direction comprises a member which is engaged with the plunger shaft surface.

5. A syringe as claimed in claim 1 wherein the plunger and restrictor mechanism are freely slidable with respect to each other over a limited range of movement, or the plunger or restrictor mechanism have relatively movable parts, whereby unlimited repeated proximal and distal cycles, of plunger head movement with respect to the barrel are permitted over a predetermined limited range which is less than said full usable range of movement of the plunger head.

6. A syringe as claimed in claim 1 wherein the said latch member comprises a separate member secured to the barrel.

7. A syringe as claimed in claim 6 wherein the said latch member comprises a projection which, in an engaged state, extends distally between the plunger shaft and the said further member on the restrictor mechanism.

8. A syringe acclaimed in claim 5 wherein the restrictor mechanism comprises two relatively movable parts, and wherein the said latch member comprises a projection extending from one of the said parts of the restrictor mechanism.

9. A syringe as claimed in claim 7 wherein the said projection extends between the plunger or barrel surface and the said further member on the other of the said two parts of the restrictor mechanism.

10. A syringe as claimed in claim 1 wherein the said latch member comprises a separate member secured on or adjacent the proximal side of the plunger head, which in an engaged state extends between the barrel interior wall and a portion of the restrictor mechanism.