Doubtless physiologists and pathologists are not invariably at one as regards the proper characters of the two, and subdivisions into continuous tonic contractions as opposed to intermittent tonic contractions have been deemed necessary; but without burdening the subject with a plethora of detail, we think it simple, suggestive, and clinically satisfactory to uphold Willis's generalisations and to enlist their help in the exposition of the tics. Hence, unless under special circumstances, we consider recourse to the epithet "convulsive" superfluous, and we shall employ the word tic by itself, except when there may be occasion to indicate the form of muscular contraction. The gain in conciseness is not likely to be neutralised by any loss of precision. From our rapid survey of the vicissitudes through which the tic has passed, we may profitably gather one or two lessons. In so far as is compatible with its nature, the schematisation of tic is indispensable. The inevitable variability of the personal factor and the absence of a real breach of continuity between any two essentially differing morbid affections ought not to deter us from the attempt to project a line of demarcation between them. Natural science is pledged to the labour of differentiation. It is the glory of Charcot's alternately synthetic and analytic work to have demonstrated the value of this method in the sphere of neuropathology. At the same time, the wisdom of attaching only a provisional importance to any scheme and of welcoming possible modification is of course self-evident. Inexact and undiscriminating inference may be a stumbling-block in the path of progress and inimical to the cultivation of the faculty of observation. Further, inaccuracy of definition not only exaggerates the liability to misunderstanding, but has sometimes also the disadvantage of promoting an illusory belief in the possession of the truth. CHAPTER III THE PATHOGENY OF TIC TIC AND SPASM Our study of tic can be approached only after a preliminary understanding as to the meaning of two words too frequently confounded even in scientific literature--_tic_ and _spasm_. Let us explain, then, once for all, exactly what we intend by the latter.