Fig. 8. Demonstration of the law of equivalent innervation (Hering's law) applicable to lid levators. A. A patient with myasthenia attempts to overcome right ptosis. Increased -levator innervation results in contralateral lid retraction (arrow). Note also elevated brows resulting from frontalis effort. B. After administration of edrophonium chloride (Tensilon), right ptosis is relieved and the left fissure narrows. Note frontalis relaxation with descent of the brows.