In the latter case, the motor reaction is called into being under the influence of an idea, normal or pathological, which eventually ceases to operate, and by virtue of the same pathogenic mechanism the act remains, inopportune and exaggerated. The objective manifestation of tic is a clonic or tonic convulsive movement, an anomaly by excess of muscular contraction. In the clonic variety there are undue rapidity and increased frequency of the movements. In the tonic variety, the duration of the contraction is prolonged. The intensity of the movements, likewise, is abnormal in degree. In spite of these disfigurations, so to speak, of the original movement, it is practically always possible to detect in them co-ordination and purpose, the cause and the significance of which ought to become the object of our search. The motor disorder can never be reduced to mere fibrillation, nor indeed to fascicular contraction unless in some one muscle different bundles have different physiological attributes. It is usual for several muscles to be concerned, and their anatomical nerve supply may be from separate sources. Like ordinary functional motor acts, tics are distinguished by co-ordination of muscular contraction and repetition; they are preceded by a desire for their execution, and succeeded by a feeling of satisfaction. These features, however, are carried to excess. In addition, the functional act is inapposite, sometimes even harmful; it may be described as a parasite function. The muscular contractions follow each other at irregular intervals; they come in attacks, which, it is true, are highly variable in frequency, duration, and degree. Volition and attention exercise a restraining influence on the motor phenomena, but repression is accompanied by malaise, sometimes by actual anguish. Distraction suspends the activity of tic; physical fatigue and emotion are calculated to arouse it. Tics always disappear in sleep. They are unaccompanied by any alteration in sensation, in the reflexes, or in the trophic functions. They are not associated with pain. * * * * * In this general way we have indicated the distinctive features of tic, and we may take the opportunity to remind ourselves of their extreme variability. In discussing the question of diagnosis, we shall have occasion to emphasise the importance of _fruste_, atypical, and transitional cases, not because we think they can be systematised as yet, but because they may be capable of new pathogenic interpretations which we cannot afford _a priori_ to set aside. We venture to believe that tic has a clinical individuality of its own which we have tried to portray, and we go so far as to say that an appreciation of the points we have touched on will prove of service in matters of diagnosis. CHAPTER XVI DIAGNOSIS TICS AND STEREOTYPED ACTS