Thus experience shows that the education of the right upper limb is reflected on the left upper limb, although the subject may be sublimely ignorant of the fact. But though this influence be latent, it is none the less real, and may prove of service if occasion arise. Weber, Fechner, and Féré[234] have all devoted attention to this subject. From the therapeutic point of view, considerable significance attaches to these facts. Temporary disablement of the right arm, such as follows fracture or arthritis or writers' cramp, need not be disconcerting, for the patient can proceed to utilise the faculty for mirror writing which his left hand has unconsciously acquired. In all affections which are accompanied by troubles of motility it is an excellent plan to apply the prescribed muscular exercises to both sides of the body, and the regularity with which they are performed on the sound side will have a corrective influence on the mirror movements of the affected side. We assume, of course, that there is no irremediable destructive lesion which interferes with the continuity of paths joining functional centres, otherwise the education of the normal limbs could not be expected to produce any beneficial effect on the other. It is especially in motor disorders of functional origin that mirror movements prove useful, and the frequent unilaterality of these disorders readily allows of the institution of a re-educative mirror drill. Speaking generally, the faculty of writing supplies us with the best means of attaining our end, for the variety of exercises it offers is likely to rivet the patient's attention, and he has proofs of his progress under his eyes. The goal in view is not, of course, the attainment of caligraphical perfection--the subjects of tic are seldom guilty of bad penmanship; but the execution of the required movements demands a voluntary constraint that cannot but be profitable. After the séances of absolute immobility, then, our custom is to set daily exercises in writing, drawing, painting, tracing, ornamentation, etc., varying the indications in accordance with individual tastes and aptitudes. At the same time, we insist on the patient's devoting both hands simultaneously to his task. It will be found advantageous to devise movements for the fingers, then for the hand, the forearm, and so on, and to instruct him in each successively. Thus, one may begin by having him make the movements in space, then with chalk on a blackboard placed vertically, then on the same placed horizontally, or on the ground; or he can be asked to trace symmetrical designs and ornaments on a wall. The essential points are that he use both arms simultaneously, symmetrically, and accurately, and that all inopportune gestures be inhibited.