Fig. 33.

is called the chiasma.

The muscles which are intended

for moving the eye, are attached to it like the bridle to ce's head. They almost all spring from the

also shows the passage of the optic nerve (2) through the bony aperture (o), and, before its exit, the cruciform shape of the combination of both optic nerves (m), which

osseous wall at the point where the optic nerve enters, and extend through the entire length of the socket to the eye-ball. There are four optic muscles which pass directly to the eye, of which one is situated above and one below, one on the outer and another on the inner side.

It is clear that the upper muscle directs the eye upwards, making it revolve upon the axis D, the lower one downwards, the inner one inwards, and the outer one outwards. Since, in ordinary vision, we always fix the same point with both eyes, we therefore move them simultaneously according to fixed laws. If we look upwards or downwards with both eyes, then the corresponding muscles are always brought into action. If, on the contrary, we look with both eyes to the right, the outer muscle is brought into action for the right eye, and the inner muscle for the left, and vice versâ. If, however, we direct our eye inwards towards a near object, then the two inner optic muscles contract; if the eyes now look at a more distant object, the two outer muscles bring the direction of the eyes more nearly parallel. We are unable to turn both eyes at the same time further apart than when their axes are parallel.

Thus we see that the contractions of optic muscles are connected in many ways. The symmetrical and similarly named muscles frequently contract simultaneously, and the opposite muscles frequently have a common action. All these combinations, however, are intended to enable us to fix the eyes upon the same point, so that the optical axes, drawn from the yellow spot through the centre of the pupil, may meet in the point upon which the eyes are fixed. We are never able so to move the

eyes that the optical axes shall not meet. We cannot, for instance, look with one eye upwards and the other downwards, or with one eye to the left and the other to the right.

Besides the muscles named there are two other oblique muscles which are attached to the eyes in an oblique direction. The position of one is superior and internal (†) and has a very curious course. For instance, it commences at the posterior aperture through which the optic nerve enters; it does not then pass forwards directly to the eye, but through a ring, u, like a cord which runs over a pulley, then turns round and is attached obliquely to the upper surface of the eye-ball. The second oblique muscle is situated upon the lower side of the eye-ball, and is not shown in the figure. It commences from the inner wall in the socket, passes onwards under the eye, and is attached (at r) opposite to the upper oblique muscle. The two oblique muscles give the eye the power of performing movements which are impossible with the aid of direct muscles alone. It is easily seen in the figure that the oblique muscles can roll the eyes round an axis (B), which approximates to the optic axis, in opposite directions.

The various directions in which the eye can be moved by means of the combined activity of the muscles named, and the precision of the motions described, not only allows the picture of the outside world to be depicted upon definite parts of the retina, but also gives expression and life to our countenance. It is chiefly the eye which betrays in our face the state of our mind and thoughts; and this is done, for the most part, by the moved position of the eyeball, associated with which

men

are, of course, the action of the facial muscles, of the eyelids, as well as the power possessed by the eye of accommodating itself to a change of circumstance. A troubled look lowers the eyes; an animated one raises them; and thus the mind, while it derives mental nourishment from without through the eye, reveals its inner actions through the same organ.

Monocular vision is incomplete. The entire field of vision is depicted on the retina as a plane surface, or like a picture, without presenting any means of distinguishing the various distances of the objects from our eye. With a single eye we only, in reality, perceive a bright surface with different lights, shadows and colours, upon which we see objects in a single plane. But practice gives us indirect means of distinguishing the distance of objects. Objects whose size we know, we should consider distant if they looked small, and near if they looked large. In monocular vision we also use the laws of perspective, from which objects gain the appearance of solidity. Again, we have to adjust our sight much more to see distinctly objects which are near, than when they are distant, and by this means we are enabled to determine their distance from us. But in reality, with one eye we still see merely a plane surface, we obtain no idea of solidity, for it is only the experience gained in life which impresses upon us the fact that we have to do with a world of space. Where this experience fails us, we not unfrequently, in monocular vision, fall into extraordinary illusions. For instance, if we are looking at the sky, and a small insect flies past, so near to one eye that it is not seen by the other, we imagine we have seen a great bird in the sky. The following experiment