The muscles forming any one pair are called associates. Any two associates acting together will move their respective eyes in precisely the same direction and to the same extent. Thus the right superior rectus moves the eye up to the left and rotates its vertical meridian to the left; and its associate, the left inferior oblique, moves its eye up to the left and rotates its vertical meridian to the left. This likewise applies to each of the other five groups of associates. If one eye fails to keep pace with the other in executing parallel movements, diplopia ensues. If the eyes are moved in all directions and the point noted where the patient just begins to see double, we _delimit_ the field of binocular single vision. Normally, however, the two eyes maintain parallelism to the very limit of their excursion, so that diplopia occurs only at the extreme periphery of the field of vision, if at all. In fact, the field of binocular single vision usually extends not less than 40 degrees from the primary position in every direction. Each of the various parallel movements of the eye appear to be governed by a distinct nerve mechanism, there being one centre for movements to the right, one for movements to the left, one for movements up, etc. MOVEMENTS OF CONVERGENCE In order to see an object at a nearby point, the eyes have to converge—a movement affected by a simultaneous and equal contraction of both internal recti. This movement may be combined with a vertical, lateral or oblique parallel movement. Thus, when we wish to look at a near object situated twenty degrees to our right, we first turn both eyes twenty degrees to the right, then converge both equally, turning the left a little more to the right and the right a little back toward the left. Convergence is governed by a distinct mechanism of the nerves, the source of which has not been determined. MOVEMENTS OF DIVERGENCE In passing from a position of convergence to a position of parallelism, the lines of sight separate or diverge. This movement of divergence is a simultaneous, equal contraction of both externi; or, probably, of both actions combined. The eyes may even diverge somewhat beyond parallelism, as in overcoming prisms, base in, when looking at a distant object. VERTICAL DIVERGENCE The amount by which the lines of sight can separate in a vertical direction is very limited—at most but one or two degrees. ORTHOPHORIA The term orthophoria is used to denote an absolutely normal balance of the extrinsic muscles, just as the term emmetropia denotes a normal refractive condition. They are equally rare. HETEROPHORIA The term heterophoria includes all those conditions in which there is a tendency to depart from normal balance, but which nature is able to compensate for; while the term also includes the conditions in which nature has been unequal to the task and an actual turning or squint has occurred. SUBDIVISIONS