The rule of law on which the giving in evidence the opinion of witnesses, who know nothing of the actual facts of the case, is founded, is not peculiar to medical testimony, but is as a general rule applicable to all cases where the question is one depending on skill and science in any particular department. . . . In general it is the opinion of the jury which is to govern, and this is to be formed upon the |109| proof of the facts laid before them. But some questions lie beyond the scope of the observation and experience of men in general, yet are quite within the observation and experience of those whose peculiar pursuits and profession have brought that class of facts frequently and habitually under their consideration. When, therefore, a question arises in a court of justice upon any such subject, and certain facts are proved by other witnesses, one skilled in such subject may be asked his opinion as to the character of such facts; or he may be asked his opinions on certain facts observed by himself. This is true with regard to any question of science, because persons conversant with such science have peculiar means, from a larger and more exact observation and long experience in such department of science, of drawing correct inferences from certain facts observed by themselves or testified to by other witnesses. The opinion of such witnesses is designed to aid the judgment of the jury in regard to the influence and effects of certain facts which lie out of the observation and experience of persons in general [269]. These witnesses are called “experts.” This term seems to imply both superior knowledge and practical experience in the art or profession. But generally nothing more is required to entitle one to give testimony as an expert, than that he has been educated in the particular art or profession; for persons are presumed to understand questions pertaining to their own profession or business [270]. The practice of admitting the evidence of experts is an old one: in the Roman Law they are frequently alluded to, and in the earliest Common Law reports they are spoken of as of established usage. Says Saunders, J., “and first I grant that if matters arise in our law which |110| concern other sciences or faculties we commonly apply for the aid of that science or faculty which it concerns. In a case of mayhem the defendant prayed the court that the wound might be examined, on which a writ was issued to the sheriff to cause to come “_medicos chirurgos de melioribus London. ad informandum Dominum regem et curiam de his quæ eis exparte Domini Regis injungerentur_ [271].”