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.

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.