8. We must now return to the consideration of the manner in which the disturbance of the brain may affect the mind. The brain is a storehouse of records of things formerly noted there by the imagination, either as the results of sense perception or of arbitrary combinations of phantasms; it is a library of facts and fancies. And these are not single, but grouped together, so that when one is stirred it will arouse others as well. When the brain is affected, whether by an acute or a chronic derangement, its images may become so disordered that records of mere imaginations get mixed up with records of real perceptions in inextricable confusion. You may have had occasion to notice the process in the case of a man who is becoming intoxicated and then passes on to _mania_ or _delirium tremens_: he gradually proceeds to mix up brain-pictures with realities, and after a while he speaks and acts like a very crazy man. He is in a kind of dream; his imaginations are wild and disconnected, his language is incoherent. The delirium arising from violent fevers, for instance from typhoid fever, is very similar to that arising from the excessive use of intoxicants and narcotics; similar in these respects; that the mania is only temporary, and that the exciting cause is not altogether unknown. The _bacilli_ of the infection, like the alcohol, the opium, the morphine, or other drugs, are accountable for the disordered action of the brain. But I do not pretend to know, nor do medical writers generally pretend to understand, _how_ the poison, or whatever causes the disease, gets to affect the brain. Does it do so directly, or by means of the alteration it causes in the whole nervous system or in the blood? We do not know; nor does it matter for the purposes of Medical Jurisprudence. IV. The questions with which the courts of justice, the lawyers, and the expert witnesses are concerned are these: Is the man really insane? Or was he insane at a given time when he performed a certain civil or criminal act? Is he now, or was he then, so far controlled by his mental unsoundness as to be incapable of acting like a rational being accountable for his actions? Even if he is now, or was then, a monomaniac, can the deed in question be traceable to his monomania as to its real cause? 1. When we know that a man is suffering from a fever, or has been drinking to excess, or has been addicted to the use of morphine, opium, cocaine or to similar deplorable practices, it is then easy enough to conclude from this that he is not in his right senses; knowing the cause, we can fairly estimate the effect. But in many cases of delusional insanity the cause is hidden; neither pulse nor other medical test betrays it. Whether the mind is sane or not is then to be found out from the man's words and actions; and these may be affected for a purpose: he may play the fool to escape punishment.