The ocular globe is put in motion in the orbit by six muscles, grouped two by two, which raise or lower the eye, turn it inward or outward, or on its anteroposterior axis. In these movements the centre of the globe is immovable, and the eye moves around its transverse and vertical diameters. These three orders of movements are independent of each other, and may be made singly, or in combination, in such a manner as to direct the pupil towards all points of the circumference of the orbit. The straight, superior, inferior, external, and internal muscles move it upward, downward, inward, and outward, and their successive action gives it a movement of circumduction. The two oblique muscles turn the eye on its antero-posterior axis, in such a manner as always to maintain the horizontal position of its transverse diameter, when the head or the body inclines to the right or the left. All these muscles take a direct or indirect part in every movement of the eye; if looking up or down, for example, the straight, superior, or inferior acts alone; the other muscles assure the movement, and" confine it to the transverse axis. Such is the perfection of this mechanism, that the cornea is raised or lowered without the least lateral deviation, like the objective of a meridian glass; and the eye perceives by this succession of movements if the image of a line on the retina deviates 0.00002 of an inch from the vertical.
The eyelids follow the movements of the globe when it is raised or lowered, obeying the action of the muscles of which they receive the aponeurotic prolongations.
The movements of the two eyes are always symmetrical, and of the same kind; both are raised or lowered at once, directed to right or left, or around their axes; they can be turned inwards simultaneously to see an object very near at hand, or slightly outwards, when they turn from such a point to one in the. distance. Even when one eye is closed, the globe turns in the same direction as that of the open eye. This unity and variety of movement contribute to make the eye the most important feature of the physiognomy.