The matter of introducing some such practice into this country has been agitated of late, and may by and by lead to beneficial results. Dr. Shrady has taken steps to promote this object by striving to have a law enacted by the New York legislature providing for the regulation of expert medical testimony in jury trials. According to his plan, once such a commission has been established, the court is to send the medical issue to these experts, just as it sends other issues to special juries to be decided. The regular petit jury will then decide only upon the facts constituting the crime. This would do away with special pleas of insanity before a jury that knows little or nothing about the nature of the disease, and whose sympathies may readily be worked upon by shrewd lawyers to render a verdict of acquittal. As things are now, the medical expert, summoned to testify in a case of contested sanity or insanity of mind, ought to rise above minor considerations, and promote the cause of justice, by giving all the valuable information that his profession enables him to acquire on the very difficult subject of mental unsoundness. 6. For this purpose, he must be skilled in three departments of science. (_a_) In _law_--sufficiently to understand what are considered by the courts as characteristic marks of an insane mind, and what amount of sanity the courts require to hold a culprit responsible for his crime or a contract valid in its effects. (_b_) In _psychology_--to such an extent that the expert witness can speak analytically and correctly as to the properties and actions of the human mind. (_c_) In _medicine_--so far as concerns the treatment of the insane, and the understanding of their peculiarities, so as to reason from them by induction to the real condition of the client's or patient's mind. But the main requisite for an expert witness is to understand clearly in what insanity properly consists, and how far it ought to excuse an insane man from bearing the consequences of his acts. III. This two-fold knowledge is obtained by the psychological study of insanity, on which study we are now to enter, and it is the principal point in this whole matter. Insanity means a want of soundness; he is insane whose mind is not sound, but is deranged, and therefore, like a machine out of order, it cannot properly perform its specific task, namely, to know the truth of things. An insane man cannot judge rightly. 1. Insanity takes various forms, which may be reduced to two kinds, with the doubtful addition of a third kind, namely, moral insanity, of which we shall speak in our next lecture. The first kind consists in the total want or gross torpor of mental activity. When there is a total, or nearly total, eclipse of the intellect, the disease is called _idiocy_, the state of an idiot. When there is an abnormally low grade of the reasoning power, it is styled _imbecility_. The failure or decay of reason in old age is called _dotage_.