I. PHYSIOLOGIC ACTION OF THE VERTICAL MOTOR MUSCLES The disturbances occurring in the sphere of action of the vertical motor muscles of the eyes present such numerous and manifold problems to the ophthalmologist and so often give rise to unexpected and unpleasant therapeutic failures that many ophthalmologists dislike to meddle with such conditions and refer the patients to other specialists from the very beginning. There are two reasons for this. While the function of the lateral muscles of the eyes is easily comprehensible under normal and abnormal conditions and the rules for diagnosis and appropriate treatment of their disturbances may be expressed in a few sentences, the correct understanding of even the normal function of the vertical motor muscles presupposes an accurate knowledge of their anatomic and especially of their physiologic characteristics, which can be appreciated only by some one who is thoroughly familiar with the fundamental laws