A connecting device including three elements, namely: a core which is housed in an inner part and an outer part. The core includes a head having a truncated-cone-shaped part and a rod having, for example, one of the two sides of a collar connected thereto, the other side being connected to the outer part. The inner part is snap engaged in the outer part and is simultaneously detached from the core. The head is shaped such that, in a closed configuration, the head inhibits the engagement element such that the device locks when the rod is pulled.