a stop position in which the portion of the tube 11 situated in the neighborhood of the end 17 is clamped around by the wall which delimits the portion 53 this clamping round provides at the same time the stop for the pushing in of the straw into the sheath and the liquid tightness between the straw and the sheath the outside surface 40 of the tail 33 will now be described in more detail with the aid of fig3 and 4 on the side turned towards the distal end which side can be seen to the right in fig2 to 4 a straight surface 60 that is to say a surface oriented according a transverse plane which projects from a trough surface 61 and on the side turned towards the proximal end which side can be seen to the left in fig2 to 4 a surface 62 inclined inwardly and towards the proximal end here the trough surfaces 61 have the same diameter and the inclined surfaces 62 have the same inclination the inclined surface 62 of the ribs 41 42 and 43 is joined to its base the part of smallest diameter situated on the opposite side to the straight surface.