Medical syringe The present invention relates to a medical syringe made up 6£ elements allowing one injection but preventing the use of the same syringe for any subsequent injection, In brief, this' is a non-reusable syringe, which self-destructs without special manipulation by the operator. Naturally, for obvious reasons, hospital and medical environments have for a long time been using disposable syringes which are used once only. However these environments are not the only ones to use syringes and trusting the user's self-discipline alone to ensure once-only use is far from adequate in the real world. For example it is well-known, according to WHO studies, that in developing countries so-called disposable syringes are often re-used up to 50 times. That is why numerous proposals have been made to make syringes which, through the way in which they are designed or constructed, physically prevent their being used a second time. The choice of the means to be adopted to this end is very limited. In fact there is really only one way, namely fitting the chamber of the syringe, basically the head of the piston, with a provisional blocking unit which ensures that the chamber is sealed for the first injection but which is definitively de-activated once the first injection has been made. The simplest version of this design is described in patent application FR-A-2, 606, 643, relating to a syringe with an eccentric nozzle, where the head of the piston is equipped with an orifice, initially stopped with a moveable cork which is ejected from the chamber when it encounters a centre punch when the piston reaches the end of its path and the chamber reaches its minimum volume. More sophisticated proposals have also been made, in particular using a membrane held by the .piston, which is then either displaced by the pressure of the liquid or punctured at the end of the first injection. Such proposals are to be found in European Patent 0345159 and in Patent application FR-A-l, 606, 643. : The number and variety of proposals of the prior art might suggest that the problem has been fully dealt with. However this is not so. In fact it is not sufficient to design a syringe which can function on this principle or one of the many variants^ thereof, it then has to be produced by an industrial process which entails a very low cost, in this respect it should be remembered that the"industrial manufacture of syringes has to meet two requirements: first of all the quantities are truly astronomical and amount to hundreds of thousands of units in order to achieve simply a modest cost in this context. Secondly, the syringes must be packed in high grade sterile packaging, in spite of the enormous manufacturing output. It is no doubt due to the fact that they did not comply with these two conditions that the proposals of the prior art have not led to any significant production, whereas the need is absolutely well-known and absolutely urgent. It is from this angle that the criticism of the previous proposals should be made, since it is indeed the case that these proposals do not meet the needs which the world is actually experiencing. In order for a non-reusable syringe to truly meet the need, it is necessary for the unit sale price to be at least comparable with that of a conventional disposable syringe, but it is also necessary for it to be equally reliable. It would actually be ridiculous to allow major production initiatives if the physically unique nature of the use were not guaranteed and remained uncertain, as is the case with the proposals of the prior art. The use of a membrane is disqualified in an industrial context because these have to be plastic articles and therefore their elasticity is an uncontrollable parameter, in particular over a period of time. Therefore one cannot guarantee stable properties and thus one would be exposed either to the impossibility of making the first injection properly or to the uncertainty of preventing the second injection in all cases. For example in the case of the syringe proposed in European patent 0345159, because there is a flexible membrane at the very end of the piston, the activation of the device is commanded by the nature of the pressure exercised on the liquid to be injected. The least drop in pressure may permanently disrupt the ongoing injection. i The use of an ejected cork is also to be ruled out because it is a plastic article inserted in another and therefore one cannot truly claim to be in control of the moment when the two parts will split up. In other words, the cork may well pop out during the injection simply due to the pressure of the liquid, and likewise it may be ejected in the opposite direction when the liquid is sucked into the syringe. In any case nobody can guarantee that it will not pop out and this is a disadvantage. The aim of the present invention is thus to propose a non-reusable syringe, whose constituent elements, and the properties thereof in combination with one another, allow on the one hand economical and reliable industrial production and, on the other hand, guarantee, through the controllable mechanical properties of the aforesaid elements, that it really can be used only once. To this end, the present invention relates to a medical syringe comprising a tubular body, in which a piston slides, the piston having an opening, a blocking unit blocking said opening before the syringe is used and this blocking unit arranged so as to cooperate, when the piston reaches the end of its path, with a counter-piece integral with the tubular body of the syringe, so as to expel said blocking unit out of the opening in such