of the liquid under the action of a thrust by the user on rod 8 represented by arrow p said deviation surfaces excert forces outwardly due to the approach of said piston and said rod to cause said cutting members to cut said wall in the embodiment shown in fig6 and 7 the deviation surfaces in the form of inclined ramps may advantageously be constituted by the bottom of a groove made in said rod 17 the sides of said groove serving to guide the flexible fingers during the relative movements between said piston and said rod 8 fig8 shows a variant according to the invention in which the cutting members are constituted by knurls 30 particularly for when said barrel is made of a breakable material such as glass plastics materials of polycarbonate type which under the pressure exerted by said knurls according to the same principles as developed for the other embodiments will break into fragments or burst in this embodiment the cylindrical barrel may be covered on the outside with a supple film which forms a jacket avoiding dispersion of the fragments resulting from the breakage the breakage advantageously occurs in the zone of least resistance 3 constituted by a groove made in the barrel said groove being filled with a supple resin which avoids the dispersion of the fragments resulting from the breakage fig9 shows the process of manufacturing the.