HELMHOLTZ IN HEIDELBERG 



this way the effect is as if the retinae were brought 

 together, and the one were placed behind the other. 

 In understanding vision with one eye there is no 

 special difficulty. The globe might rotate round three 

 possible axes, a vertical, a horizontal, and an antero- 

 posterior. Movements are affected by four straight 

 muscles (rect'i) and two oblique. The four straight 

 muscles (rectus superior, rectus inferior, rectus externus, 

 rectus internus) arise from the back of the orbit, and 

 pass forwards to their insertion in the front part of the 

 eyeball, or its equator, if we regard the anterior and 

 posterior ends of the globe as the poles. The two 

 obliques (while one of them also originates at the back 

 of the orbit) come as it were from the nasal side, the 

 one goes above the eyeball and the other below, and 

 both are inserted into the eyeball on the temporal side, 

 the superior oblique above, and the inferior oblique 

 below. The six muscles work in pairs. Thus the 

 internal and external recti turn the eye round the 

 vertical axis, so that the line of vision is directed to 

 the right or left. The superior and inferior recti turn 

 the eye round the horizontal axis, and thus the line 

 of vision is raised or lowered. The oblique muscles 

 turn the eye round an axis passing through the centre 

 of the eye to the back of the head, so that the superior 

 oblique lowers while the inferior oblique raises the 

 visual line. Helmholtz was the first to discover that 

 the oblique muscles, in certain circumstances, cause a 

 slight rotation of the eyeball round the visual line 

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