A classic instance of this variety of tic is Ros., long known at Bicêtre as "the waltzer." CHAPTER XV THE DISTINCTIVE FEATURES OF TIC We are scarcely inclined to believe in the possibility of condensing into an adequately concise and adequately precise formula our conception of tic, or at least all the notions which contribute to it. Because most authors feel it incumbent on them to fall in with this nosographical custom, definitions have been proposed whose brevity only serves to confuse the issue. Opinion on the interpretation of certain words which concern our subject is far from being unanimous, and, as we remarked at the outset, accuracy in our terminology is urgently called for. This has been our reason for preceding our definitions by the results of clinical observation and pathogenic analysis. Our idea of tic, however, may be couched in the following terms: _A tic is a co-ordinated purposive act, provoked in the first instance by some external cause or by an idea; repetition leads to its becoming habitual, and finally to its involuntary reproduction without cause and for no purpose, at the same time as its form, intensity, and frequency are exaggerated; it thus assumes the characters of a convulsive movement, inopportune and excessive; its execution is often preceded by an irresistible impulse, its suppression associated with malaise. The effect of distraction or of volitional effort is to diminish its activity; in sleep it disappears. It occurs in predisposed individuals, who usually show other indications of mental instability._[167] We are in a position, now, to elaborate the details of this definition. Tic is a psychomotor affection, and there are two inseparable elements in its constitution, a mental defect and a motor defect. The prevailing mental defect is impairment of volition, which takes the form either of debility or of versatility of the will. This being characteristic of the mind of the child, its continuance in spite of years argues a partial arrest of psychical development. Hence the epithet infantile may be employed to qualify the patient's mental state. Other psychical troubles, which similarly are anomalies of volition, may be superadded, in particular impulsions and obsessions. Speaking generally, a certain degree of mental instability is a distinguishing feature of the patient with tic. The defect of motility consists at first in the provocation of a motor reaction by some external cause, or by an idea. In the former case, the reaction is the cortical response to a peripheral stimulus, and its logical execution becomes by dint of repetition habitual and automatic. With the disappearance of the stimulus it continues to manifest itself, without cause and for no purpose, in which circumstances the feebleness of the inhibitory power of the will is revealed.