The custody, care, and treatment of a defendant committed under the provisions of this Title, and mental examinations and reports subsequent thereto by physicians on the medical staff of the institution, shall be without cost to the parish from which the defendant was committed. When a physician on the medical staff of the institution is called to testify at a hearing or trial concerning a mental examination made by him, he shall be entitled to a reasonable expert witness fee to be fixed by the court, and to reimbursement for traveling expenses in conformity with law, with such fees and expenses to be paid by the parish where the prosecution was instituted.