When one eye looks at a distant object the other is also directed to it, so that the lines of sight of the two eyes are parallel; if the distant object is moved about, the lines remain parallel, one moving as fast and as far as the other. These parallel movements of the two eyes are executed with considerable freedom in all directions, either eye being able to move readily to the right, left, up, down, or obliquely, provided the other eye moves precisely with it. In executing any parallel movement, each eye is acted upon by at least three and sometimes by as many as five muscles. At times, but one of these muscles is required to produce any _great_ movement of the eye, the others simply serving to steady it in its course. Thus when we look up to the right, although there are five muscles really acting upon each eye, the right eye is moved mainly by the _external_ rectus and the left eye by the _internal_ rectus. Similarly, when we look up and to the right, although other muscles take part, the superior rectus is the chief muscle that moves the right eye up, and the external rectus the chief one that moves it to the right; while for the left eye the inferior oblique and the internal rectus are the efficient muscles. A careful study of the action of the individual muscles will make it clear that these facts hold good for each of the cardinal directions of the gaze. Furthermore, if we attentively consider the action of the twelve muscles moving the two eyes, we see that they may be divided into three groups, _viz._; four _lateral rotators_, four _elevators_ and four _depressors_. LATERAL ROTATORS Right rotators } { Left rotators L. Internal rectus } to { R. Internal rectus R. External rectus } { L. External rectus ELEVATORS Right-handed elevators } { Left-handed elevators (acting mainly when the } { (acting mainly when the eyes are directed to the } to { eyes are directed to the right) } { left) R. Superior rectus } { R. Inferior oblique L. Inferior oblique } { L. Superior rectus DEPRESSORS Right-handed depressors } { Left-handed depressors (acting mainly when the } { (acting mainly when the eyes are directed to the } { eyes are directed to the right) } to { left) R. Inferior oblique } { R. Superior oblique L. Superior oblique } { L. Inferior rectus. Each group, it will be seen, comprises _two_ pairs of muscles; one pair acting solely when the eyes are directed to the right, the other when they are directed to the left. It will further be noted that of the two muscles constituting any one pair, one is situated in the right eye, the other in the left. EYE ASSOCIATES