The localisation of the motor reaction in cases of tic is essentially physiological. In rare instances its sphere may be limited to a single muscle, if one muscle only be requisitioned for the performance of a functional act; but it is very much more usual to find several muscles contributing, whose synergic contractions fashion the movement of which the tic is a caricature. If the same effect is yielded by the action of either of two different muscles or groups of muscles, as in rotation of the head, and if one be hindered from fulfilling its function, the incidence of a tic originally located in it will promptly be transferred to the other. This is the explanation of the persistence of rotatory tics after exclusion of the sternomastoids by surgical means. Two symmetrical muscles may be affected, as in tics of blinking and of affirmation, or a median muscle, such as the orbicularis oris. Much more frequently the tic is unilateral in its distribution, as, for instance, when it involves the face; in this respect its figuration as a functional disturbance is well exemplified, for expressional movements of the face are normally bilateral. A tic may settle itself on two mutually antagonistic muscles, and manifest its presence in the immobilisation of a limb or segment of a limb; or only a portion of a muscle may contract, as in the case of the deltoid or trapezius, which are composed of bundles anatomically associated but physiologically independent, and so capable of being functionally differentiated by voluntary education. Fibrillary contraction and tic have nothing in common. Inasmuch as the muscles concerned are under voluntary control, and their contractions such as the will can effect, it follows that with adequate practice the movement of a tic can always be imitated, and in predisposed soil imitation tics may thus take root; it is not always feasible, on the other hand, to counterfeit a spasm. Several functional muscular territories may be simultaneously affected, and several tics may follow one another in quick succession, the duration of any one tic on any one site being a more or less varying quantity. We have already noted the occurrence of variable tics. They appear one day to disappear a few days later, and reappear again after another space. Weeks or months may elapse without any vestige of them, until they suddenly break forth again unheralded. As a general though not absolute rule, the younger the patient, the less stable his tics. Occasionally they are isolated, limited, and stationary, one of the most frequent of this kind being a tic of blinking, but the intimate alliance between the motor troubles and the mental level of the subject helps to explain why these tics of children are so changeable.