referring more specifically to the drawings , a first form of syringe in accordance with the invention is indicated generally at 10 in fig1 - 9 and 11 - 15 . in this form of the invention , a conventional syringe barrel 11 has a forward end 12 with a suitable means , such as a luer lock adapter 13 for attachment of a needle 14 , and an open rearward end 15 . an elongate plunger 16 is reciprocable in the barrel between a forward position inserted fully into the barrel , and a rearward position retracted or withdrawn in the barrel , and has a forward end 17 and rearward end 18 . the plunger has a generally h - shaped or i - beam - shaped transverse cross - section , with a central web 19 and oppositely extending flanges 20 and 21 along each side edge of the web . the rearward end of the plunger has a radially enlarged , generally bow - tie - shaped operating flange 22 which may be gripped with the fingers and used to reciprocate the plunger in the barrel . the forward end of the plunger has a reduced transverse dimension and defines an elongate , forwardly extending attaching post 23 with a plurality of radially outwardly projecting latching arms 24 integrally pivotally connected to the post at hinge areas 25 . four such latching arms are shown in the specific example described herein , but it is contemplated that a different number of arms could be used , if desired . each arm includes a thickened outer end portion defining detents 26 that are arranged in outwardly spaced , confronting relationship to a radially outwardly projecting retaining ring 27 formed on the post forwardly of the point of attachment of the arms to the post . a piston 30 is carried on the post 23 at the forward end of the plunger , and in the preferred construction shown in fig1 - 9 and 11 - 15 is made of a synthetic plastic material . this plastic piston has a pair of oppositely axially projecting sealing flanges 31 and 32 , each flared radially outwardly and having a radially enlarged sealing bead 33 thereon for effecting a sliding seal with the inner surface of the barrel . thus , during forward motion of the piston in the barrel , pressure of fluid in the barrel acting under the sealing flange 31 causes that flange to expand radially outwardly , making a tight sliding seal with the inner surface of the barrel . conversely , rearward movement of the piston in the barrel causes lowered pressure in the forward end of the barrel to pull the sealing flange 32 outwardly to effect a tight sliding seal with the inner surface of the barrel . the piston is held on the forward end of the plunger by a plurality of detents 34 on the end of the piston adjacent the plunger , spaced radially inwardly from the sealing flange 32 , and clamped between the latching arms 24 and retaining ring 27 . these detents are molded with a natural , unbiased position as shown in fig7 and 14 , spaced radially outwardly out of contact with the retaining ring 27 , and in the operative position of the invention are held inwardly behind the retaining ring by the latching arms 24 . as seen best in fig3 and 10 - 15 , the outer ends of the arms 24 , the retaining ring 27 and the detents 34 are uniquely shaped to cooperate with one another and with the inner surface of the barrel during reciprocal movement of the plunger in the barrel to either maintain the piston latched to the plunger , or to disengage the piston from the plunger . with particular reference to fig3 and 12 , a first of these surfaces 40 defines a relatively narrow annular cylindrical band around the outer perimeter of the latching arms , which is parallel to the inner surface of the barrel and is adapted to slide along the inner barrel surface when the arms are in their normal , operatively latched position with respect to the detents on the piston . thus , with the piston beginning in its forward - most position at the forward end of the syringe , as seen in fig1 , this surface 40 is in parallel , sliding contact with the inner surface of the barrel . at the same time , a second , latching surface 41 on a radially inner portion of each latching arm is in parallel , mating contact with a complemental latching surface 42 on an upper outer end portion of the detents 34 on the piston to hold the detents inwardly behind the retaining ring 27 and therefore latch the piston to the plunger , as shown in fig3 and 12 . after the piston and plunger have been withdrawn in the barrel , and forward motion thereof is then initiated , the frictional drag between the outer ends of the latching arms and the inner surface of the barrel causes the arms to pivot rearwardly , as depicted in fig1 , through an over - center position to a rearwardly flexed inoperative position as shown in fig1 and 15 and in dot - and - dash lines in the left hand side of fig3 . the over center action results from the difference in diameter of the latching arms in comparison with the diameter of the inner surface of the barrel . thus , when the arms are in their latched , operative position as shown in full lines in fig3 the first surface 40 is on essentially the same diameter as the inner diameter of the barrel , and this surface is in close , sliding contact with the inner surface of the barrel . however , when the arms pivot rearwardly upon forward movement of the piston in the barrel , the outer ends thereof swing through an arc that places the outer ends of the arms on a greater diameter than the diameter of the inner surface of the barrel . continued forward movement of the piston in the barrel results in the arms pivoting to their unlatched position shown in dot - and - dash lines at the left hand side of fig3 and in full lines in fig1 and 15 . in this position , a third surface 43 on the outer end of the latching arms is in parallel , sliding contact with the inner surface of the barrel . this surface 43 has substantial width in relationship to the first surface 40 , and maintains the latching arms in this unlatched position , regardless of the direction of motion of the piston in the cylinder . with reference to fig3 and 14 , the operating relationship between a fourth surface 44 on the underside of the latching arm and the upper end of the detents 34 will become apparent . the upper end of the detents has a slightly tapered surface 45 that extends between a heel 46 at the radially outermost end thereof , to a nose 47 at the innermost end . thus , when the plunger and piston have been retracted in the barrel , and forward motion thereof is then initiated as shown in fig1 and 14 , the heel 46 begins pushing upwardly against surface 44 on the latching arm , and , combined with the frictional drag of the outer end of the arm against the inner surface of the barrel , begins upward flexing movement of the arm . continued movement in this direction , as depicted in fig1 , causes the nose 47 to begin sliding upwardly along a fifth surface 48 on the post immediately below the point of attachment of the arms , resulting in radially outward pivoting movement of the detents and continued upward pushing action of the heel against the surface 44 . the latching arms are thus pivoted completely through this &# 34 ; over - center &# 34 ; motion to their fully unlatched position shown in fig1 , where the end surface 45 on the detents is in parallel contact with the undersurface 44 of the latching arms , securely holding the latching arms in their unlatched position during downward movement of the plunger and piston in the barrel and providing a large contact area between the plunger and piston for pushing the piston forwardly in the barrel . when the plunger is again retracted in the barrel , as depicted in fig1 , the surfaces 43 on the outer ends of the latching arms easily slide along the inner surface of the barrel , whereby the latching arms are maintained in their unlatched position , regardless of the direction of reciprocation of the plunger in the barrel . during this subsequent withdrawal of the plunger in the barrel , the unlatched piston remains in its forwardmost , previously pushed position in the barrel , as depicted in fig1 . during assembly of the syringe of the invention , the piston is first inserted into the barrel through the open rearward end thereof , and the plunger is next inserted to bring the post and latching arms into the relationship with the piston as shown in fig1 . a special tool t is then inserted through the open end of the barrel and into contact with the latching arms , and is used to force the latching arms through their over - center position into the latched position shown in fig1 . a modification of the invention is shown at 50 in fig1 , wherein the piston 30 &# 39 ; 0 is made of rubber rather than plastic as in the previously described form of the invention . the piston itself is constructed differently in the area that it seals with the barrel , but the latching mechanism is substantially identical to that previously described . the sliding seal between the piston and the barrel in this form of the invention is accomplished by a pair of axially spaced sealing rings 51 and 52 on the outer surface of the piston at its forward and rearward ends . in addition , the post 23 &# 39 ; is substantially shorter than that previously described . otherwise , this form of the invention functions identically to the first form . in a specific example of the invention , and with the latching arms in their natural , as - molded position as shown in fig5 the first surface 40 is disposed at an angle of 15 ° relative to the longitudinal axis of the syringe , the third surface 43 is disposed at an angle of 25 ° relative to the longitudinal axis , and the fourth surface 44 is inclined 20 ° with respect to the transverse axis of the syringe . further , there are four substantially uniformly circumferentially spaced latching arms and eight substantially uniformly circumferentially spaced detents , with each latching arm arranged to press against two of the detents . this can perhaps be best understood by referring to fig4 and 6 . the syringe of the invention is simple and economical in construction , and does not require any more parts than a conventional syringe , i . e ., the barrel , plunger and piston . it is anticipated that the plastic piston version can be produced at a cost comparable with conventional syringes , which use a rubber piston but which do not have the disabling feature of the present invention . while the invention has been illustrated and described in detail herein , it is to be understood that various modifications may be made therein without departing from the spirit and scope of the invention , as defined by the appended claims .