The physician called to give evidence as an expert should understand at the outset that he is not called to express any opinion upon the merits of the case, but only on some questions of science raised by the facts proved; that he has no concern in the issue of the trial, and that whichever side calls him he is in no wise the witness—much less the advocate—of that side. He is truly an adviser of the Court, an _amicus curiæ_, rather than a party interested in the result of the trial. Balbus in his commentaries on the code says, “_Medici proprie non sunt testes, sed est magis judicium quam testimonium_.” Experts, no matter on what |116| they testify, simply supply data, as to whose competency, relevancy and weight, the Court is to judge, and as to which the Court is finally to declare the law. Where the facts testified to by experts are undisputed, and when they are the results of a particular science or art, with which such experts are familiar, then the Court accepts such facts, and declares the law that therefrom springs; where the facts are disputed then the jury is to determine where the preponderance of proof lies. But when the testimony of the expert touches either law or speculation, psychology or ethics, then such testimony is to be received as mere argument, which if admissible at all is to be treated simply as if addressed to the judgment of the Court [293]. In his examination in chief an expert may not only give his opinion itself, but also the grounds and reasons of it; in fact it has been held that it is his duty to state the reasons of his opinion and the facts on which it is based, and if it is not sustained by them it is entitled to little weight [294].