38
Anatomical Description of the Eye
[32-34.
of the orbit, and uniting and crossing with the nerve from the opposite
side in the chiasma nervorum opticorum at m. The continuations of the
optic nerve from the chiasma to the brain are known as the optic tract
(;tractus opticus). The fibres of each tractus opticus pass partly into
the optic nerve of its own side, partly into that of the opposite side,
and a small part through the tractus opticus of the opposite side back into
the brain. Some observers have also found fibres which pass from one
optic nerve through the chiasma into the other.
There are six muscles whose function is to turn the eyeball in its
socket, namely:
1. The internal rectus i, and
2. The external rectus a. Both of these originate in the vicinity of
the foramen opticum at the apex of the orbit and are inserted on the
inner (medial) and outer (lateral) sides of the eyeball, respectively.
They turn the eye around its vertical axis.
3. The superior rectus, removed from the right side of Fig. 22 in
order to show the optic nerve, marked s on the left side; and
4. The inferior rectus, which lies on the under side of the orbit, just
as the superior rectus shown in the figure lies on the upper side. These
two muscles also originate in the vicinity of the foramen opticum and
are inserted in the upper and lower sides of the eyeball. They turn it
around a horizontal axis, indicated by the line DD in Fig. 22, which
passes from the nasal side of the eye, a little to the front, to the
temporal side, a little to the back, making an angle of about 70° with
the optical axis (A) of the eye.
5. The superior oblique muscle t arises from the edge of the foramen
opticum and proceeds to the upper nasal side in the front part of the
orbit, where its tendon passes through a small pulley n (trochlea),
which is attached to the upper anterior edge of the orbit. Here it turns
at an angle and is inserted in the upper side of the eyeball at C. The
muscle exerts a pull in the direction of its tendon.
6. The inferior oblique muscle, not shown in the figure, arises from
the front nasal border of the orbit, passes under the eyeball towards the
temporal side, and is inserted in the posterior outer side of the eyeball
at v in Fig. 22. The axis of rotation (BB) of the oblique muscles of the
eye is likewise horizontal and passes from the outside in front to the
inside behind, making an angle of about 75° with the axis of rotation of
the superior and inferior recti and an angle of 35° with the optical axis
of the eye.
The optical axis of the eye may be turned in any desired direction by
the action of these six muscles combined in different ways. The eye¬
ball is capable of rotation also about the optical axis itself. As an intro-