A medical man may be called as a _common witness_, or as an _expert_ witness. In the _first_ case, he has only to state, as any other witness might do, the facts that have fallen under his observation; in the _second_, he has to interpret the facts he has himself observed, or to give his opinion on facts noticed by others. In stating his opinion, a medical witness must be prepared to back up his opinion by such reasons as may be satisfactory to the understanding of his hearers, “and this is the principal qualification of a medical witness, that he make himself _intelligible to ordinary comprehensions_.” No man is bound to give any testimony by which he may render himself liable to any criminal prosecution. (See the ruling of Bailie, J., in the case of Mr. George Patmore, tried for the murder of John Scott in a duel.) At the trial, the witness is first examined by the party who calls him: this is the examination-in-chief. He is then cross-examined by the opposite party; and, lastly, re-examined by the former party, when he is offered the privilege of explaining any discrepancies between his examination-in-chief and cross-examination, but he must not introduce any new matter, for by so doing he renders himself liable to be cross-examined on it. =The Use of Notes.=--All notes should contain a plain statement of the facts, and, to render them admissible as evidence, they must be taken _at the time_, and duly attested. From the notes prepared as before mentioned a witness may refresh his memory, but they are not accepted in its place. A witness may not read his notes as evidence, nor may he refresh his memory by documents not his own and not produced, but he may refresh his memory by looking at a document received from the accused at the time of the offence, and kept by him (Geo. Wilson, jun., Aberdeen, May 1, 1861; 4 Irv. 42). =The Use of Books.=--No witness is allowed to quote from books, or to quote the opinion of other medical men on the subject, but he may refer to facts. Sir Henry Littlejohn, in his papers on Medical Jurisprudence,[2] gives some useful hints on this subject. It appears that a medical witness, in an unguarded moment, stated that his opinion was corroborated by a distinguished member of the medical profession not engaged on the trial. The judge informed the witness that it was most irregular to have other medical men present at the dissection than those mentioned in the warrant, and that, if the witness did not feel qualified for conducting such dissections, he had better resign the post of medical inspector. [2] _Edinburgh Medical Journal_, February 1876.