The eyeballs may be regarded as spherical bodies, lying in loosely fitted sockets of connective tissue padded with fat, in which they can move or revolve freely in all directions, in a limited degree. The muscles which act directly on the eyeball are six in number. Four recti passing from the back of the orbit are attached to the eyeball, one at each side and one above and below, not far from the cornea. These move the front of the eye to the right or left, up or down respectively. Two oblique passing nearly horizontally outward, and a little backward, are attached to the upper and under surface of the eyeball respectively. These muscles can slightly rotate the eye on its anteroposterior axis, the upper one drawing the upper part of the eyeball inward, and its antagonist, the lower, drawing the lower part inward, so as to rotate the eyeball in an opposite direction round the same axis.
The internal and external recti draw the centre of the cornea to or from the median line respectively, directly opposing one another.
Fig. 236. Diagram of the direction of action of the muscles of the eyeball, which is shown by the dark lines. The axes of the rotation caused by the oblique and upper and lower recti are shown by the dotted lines. The inner and outer recti rotate the ball on its vertical axis, which is cut across. The abbreviated names of the muscles are affixed to the lines.
As the direction of the superior and inferior recti is different from that of the axis of the eyeball, they draw the outer edge of the cornea, not its centre, up and down respectively, and at the same time tend to give the eyeball a slight rotation in the same direction as the corresponding oblique muscles. The tendency to rotation is counteracted by the antagonistic oblique muscle when simple elevation or depression is formed.
Thus, pure abduction or adduction only requires the unaided action of the internal or external recti, while direct depression of the eye requires the combined action of the inferior rectus and superior oblique, and direct elevation requires the superior rectus and inferior oblique to act together. The oblique movements are accomplished by various combined coordinations of movement of the different muscles.
From the foregoing it is obvious that the simplest movements of the eye require the cooperation of different muscles.
The diagram shows the directions toward which the different muscles tend to draw the eyeball.
In the ordinary movements of both eyes more than this is necessary. Both eyes must move in the same direction at the same time, now to the right, now to the left, so that while the external rectus moves the right eye to the right side, the internal rectus moves the other eve in the same direction. The coordi-nation of the movements of the eyeball is so arranged that the contractions of the external and internal recti of opposite sides must occur together, and are called "associated movements." This associated movement has been acquired by the habit of voluntarily directing both eyes at the same object, and has gradually become involuntary, for few persons have the power of exerting control over the muscles of one eye alone.