56As regards the admissibility of the objection of illegality raised by the applicant under Article 184 of the Treaty with regard to the sickness insurance rules, it must be borne in mind that, as the Court of Justice has held (Case 92/78Simmenthal v Commission [1979] ECR 777, paragraphs 39 to 41), the objection of illegality gives expression to a general principle conferring upon any party to proceedings the right to challenge, for the purpose of obtaining the annulment of a decision of direct and individual concern to that party, the validity of previous acts of the institutions which form the legal basis of the decision which is being contested. Consequently, such objections cannot be limited to measures in the form of a regulation, the only kind mentioned in Article 184 of the Treaty, but must be interpreted broadly as including all measures of general application. The sickness insurance rules which, having been adopted in implementation of Article 72(1) of the Staff Regulations, essentially cover the reimbursement of the various sickness expenses incurred by members of the Joint Scheme, were adopted, in their original version, in 1974 by the European Community institutions in an agreement recorded on 31 October 1974 by the President of the Court of Justice; they have been amended on several occasions, most recently in 1991, the agreement of the institutions having been recorded by the President of the Court of Justice on 28 November 1991. Those rules are of a general nature, in that they apply to situations that are determined objectively and have legal effects with regard to categories of persons referred to in a general and abstract manner (judgment of the Court of Justice in Joined Cases 44/74, 46/74 and 49/74 Acton and Others v Commission [1975] ECR 383, paragraph 7, and Case 206/87Lefebvre Frère et Soeur v Commission [1989] ECR 275, paragraph 13). Consequently, although they are not in the form of a regulation, those rules may be the subject of an objection of illegality. Moreover, the Court of Justice itself has described them as ‘provisions implementing the Staff Regulations’ and has examined their compatibility with the relevant provisions of the Staff Regulations, in particular as to whether in certain respects they exceed the limits laid down by the Council in Article 72 of the Staff Regulations (Case 806/79Gerin v Commission [1980] ECR 3515, paragraph 15, and Case 339/85Brunotti v Commission [1988] ECR 1379, paragraph 13).