The rotational movements of intorsion and extorsion are necessary to keep a viewed object stable and positioned on the appropriate part of the retina as the head is tilted from side to side. To illustrate imagine standing upright and looking at an arrow that is pointing superiorly. Now tilt your head 45 degrees to the right, does the arrow move with your head? Now point at a 45 degree angle, hopefully not. Because of intorsion and extortion the arrow should still appear to be pointing superiorly. In this case the right eye has been intowarded and the left eye has been equivalently extorted so that the image falls on the same part of the retina as before the head tilt. Obviously our eye has a limited range of intorsion and extorsion. They cannot spin completely around within the orbit. And if we were to stand on our head the arrow would appear to change direction and appear to point inferiorly. Now, lets look at the movements produced by each of the extra oculum muscles. Medial and...
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