to the facts which he found present upon examination. Accordingly it is not good practice to call a physician and ask him "whether a person was in good health and free from any symptom of disease" without having first given him an opportunity to state facts upon which such a conclusion might be based.[1] In short, the opinion of the expert must be based upon facts or statements appearing in evidence in the case. This statement is, perhaps, subject to an exception, in that it is proper for the expert to express an opinion based upon statements and expressions of the patient made at the time of the examination. The extent to which this rule is applicable is the subject of some conflict, the courts of some States holding that where the examination is made after suit is commenced, not for the purpose of treatment, but merely for the purpose of information, exclamations of pain are properly excluded.[2]
It will be readily perceived that an urgent reason for the foregoing rules is that the court and jury may know the grounds and reasons upon which the expert's opinion is based, and, apparently, that the urgency of this reason may not be lost sight of, it is a recognized rule that either party may ask the expert the reasons upon which his opinion is based, or the expert himself may, with the permission of the court, give such reasons and explanations unasked by counsel.
The following general rules governing the examination of expert witnesses have been laid down by Mr. Rogers in his excellent work upon the subject of Expert Testimony:—[3]
First. "Evidence should be confined to the points in issue, and evidence of collateral facts which are incapable of affording any reasonable presumpton as to the principal matter in dispute should not be received."
Second. "Leading questions should not be asked on the direct, but may be asked on the cross-examination of a witness." This is subject to exception where the subject-matter of the evidence is complicated, and does not apply when the witness is hostile.
Third. "In England the rule is that the examination and cross-examination of a witness must relate to the facts in issue, or relevant
