Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 1. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
Anatomical Description of the Eye 
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-


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