We have already, on more than one occasion, drawn attention to the phenomena known as stereotyped acts, demonstrating their intimate kinship with the tics and the frequent difficulty of establishing a differential diagnosis. To ensure precision of ideas and of terminology, we must restrict the expression to motor disturbances in which the characters of the muscular contraction are identical with those of normal acts. On this view many motor reactions are really classifiable as stereotyped acts, and among them are those denominated by Letulle "habit tics." Stereotyped acts occur in normal individuals, and it may fairly be said there is no one but has his habitual gesture, his movement of predilection. As a matter of fact, a certain number of what Letulle calls co-ordinated tics belong to the group under consideration; others, no doubt, are genuine tics, and between the two may be found innumerable intermediate varieties. From the diagnostic standpoint the stereotyped acts that occur in the course of mental disease, of which a conscientious study has recently been made by Cahen,[168] are highly instructive. He defines them as non-convulsive, co-ordinated attitudes or movements, resembling intentional or professional acts, repeated at frequent intervals and always in the same fashion, till their conscious and voluntary performance is replaced by a degree of subconscious automatism. In the case of the insane they are secondary to some delusion, and persist though the latter may disappear. Hence the patient may be incapable of explaining his movements and attitudes, however much he may persevere in their automatic execution--an evolutionary process akin to that of the tics. A typical instance may be quoted from Séglas: B. passed under observation in 1891, suffering from delusions of persecution, and not long afterwards it was noticed that from time to time he used to come to a halt in the courtyard, gaze at the sun, and rotate his hands round an imaginary axis. The reply he vouchsafed to interrogation on this point was that he was effecting the sun's revolution. At present, however, he has sunk into a state of dementia, and while the gesture continues he is unable to furnish any explanation of it. Of course it is inadmissible to apply the term to co-ordinated acts that are neither conscious nor voluntary, such as the teeth grinding of the general paralytic, or the body oscillation of the idiot. Similarly one must differentiate them from impulsive seizures, abrupt irresistible motor explosions neither frequent nor prolonged. A distinction has been drawn between _akinetic_ (attitude) stereotyped acts and _parakinetic_ (movement) stereotyped acts. As instances of the former we may give the following: A woman reclines continuously in bed because she believes she has an infernal machine in her abdomen.