converge symmetrically from the stepped widened section with respect to the axis a of the coupling part 1 and adjoin a top part 8 c the head parts 8 a 8 b and the top part 8 c together form a head 8 with a rear end which forms a transverse detaining surface 9 due to the stepped widened section the head 8 has a width which gradually decreases from the rear end to the top the female coupling part 11 is an elongate element see in particular fig2 a 2 b and 3 having a substantially hollow end part in which a receiving space 12 for the head 8 of the male coupling part 1 is provided which is accessible via a passage 13 which is provided from the free end of the female coupling part 11 and ends in the receiving space 12 the passage 13 extends along the longitudinal axis b of the coupling part 11 and has a substantially cylindrical shape with a conically narrowing inlet 14 at the end of the coupling part 11 the edge which is situated around the outlet of the passage 13 in the receiving space see fig3 serves as a detaining means 15 for the head 8 as will be explained in the remainder of this description the receiving space 12 is provided with a cylindrical uncoupling body 16 in which a central opening 17 which is likewise cylindrical is provided this uncoupling body 16 can be displaced along the longitudinal axis b of the coupling part 11 in the receiving space 12 as is illustrated in fig4 at the head 8 a laterally protruding protuberance 8 d is provided on one of the head.