Psychiatry is that branch of neurology which treats of mental disorders and of the organic changes associated with them.

Mental disorders arrange themselves in two fundamental categories, characterized respectively by insufficiency and perversion of the psychic faculties.

Insufficiency may be either congenital or acquired. In the first case it constitutes arrest of development; in the second, psychic paralysis. When the psychic paralysis is temporary, causing a suspension, but not a destruction, of mental activity, it is spoken of as psychic inhibition; on the other hand, when it is permanently established, it constitutes mental deterioration or dementia.

Perversion of the psychic faculties may also be congenital or acquired. Various terms are applied to its manifestations, depending upon the particular function affected: hallucinations, delusions, morbid impulses, etc.

Mental diseases or psychoses are affections in which mental symptoms constitute a prominent feature. They differ from such mental infirmities as idiocy, constitutional psychopathic states and some states of dementia, in that they are expressions of active pathological processes and not of permanent and fixed alterations of the mind.

Psychic infirmity, when not congenital, occurs as the outcome of mental disease. The relation between the two conditions is analogous to that which exists between ankylosis of a joint and the arthritis which produced it; the latter is a disease, the former an infirmity.

Two general terms still remain to be defined: mental alienation and insanity. Although they are often employed indiscriminately, their meaning is not quite identical.

Etymologically, an alienated (Lat. alienus) individual is one who has become "estranged" from himself, who has loct the control of his mental activity, who, in other words, is not responsible for his acts. This definition rests upon the metaphysical conception of a free will and cannot find a place in medical science, which must be based on observation and must adhere to demonstrable facts.

It is better to adopt an essentially practical definition, as has been done by most modern psychiatrists, and to designate by the term mental alienation the entire class of pathological states in which the mental disorders, whatever their nature be otherwise, present an anti-social character. Not every individual suffering from a psychic affection is alienated. This term can be applied only to those who, on account of some mental disease or infirmity, are likely to enter into conflict with society and to find themselves, in consequence, unable to be an integral part of it.

Insanity, as a scientific term, has fallen into disuse and now retains significance mainly as a legal one. Like lunacy, it seems destined to become obsolete, for even in law it is not used without stated or implied further specification, such as incompetence necessitating the appointment of a committee of person or estate, irresponsibility in a criminal action, limited testamentary capacity, or irrational conduct necessitating commitment to an institution for treatment and custody. Thus, according to the law of the state of New York, an imbecile, epileptic, or senile dement ("dotard") cannot be committed to a state hospital unless he is at the same time insane, i.e., delusional, depressed, excited, or otherwise irrational in conduct; similarly, some cases of hysteria, neurasthenia, cerebral arteriosclerosis, or brain tumor may be declared insane and committed to an institution, and others not, depending on their manifestations from a sociolo-logical standpoint.

This Manual is divided into three parts. The first deals with general psychiatry, comprising causes, symptoms, methods of investigation, treatment, and prevention of mental disorders considered independently of the affections in which they are encountered. The second deals with special psychiatry, that is to say, with the various mental affections individually. The third consists of appendices giving technique of special diagnostic procedures.