The expert witness differs essentially from the ordinary witness in at least two particulars; first, in that the field of his testimony is outside the range of ordinary knowledge and experience; and, secondly, in that his testimony in the great majority of cases is in the form of opinions or conclusions that are deemed necessary for the proper guidance of the jury. It goes without saying that the 'lawyer who undertakes the examination of the expert should have such familiarity with the subject of inquiry as will enable him to develop it through the expert logically and clearly, but unfortunately it does not go without saying that this is generally the case.
Hutchins, Harry B. "Examination of the Medical Expert." Mich. L. Rev. 3 (1905): 520-540, 597-616.