Professional service fees may be charged by physicians, dentists and nurses for personal services rendered to private patients in the Government hospitals. If Government supplies are used, their actual cost shall be billed monthly by the Government hospital and paid for by the private practitioner monthly. The term “private patients” as used in this section means any patient who specifically requests and contracts for the services of a particular physician, dentist or nurse. Government physicians, dentists and nurses may charge fees for services rendered outside their regular duties.