CH. LVIII.] POSITIONS OF THE EYEBALLS 851 



The three axes around which the movements occur are : 



1. The visual or antero-posterior axis. (A P, fig. 537.) 



2. The transverse axis, which connects the points of rotation of 

 the two eyes. (Tr, fig. 537.) 



3. The vertical axis, which passes at right angles to the other 

 two axes through their point of intersection. 



The line which connects the fixed point in the outer world at 

 which the eye is looking to the point of rotation is called the visual 



FIG. 537. Diagram of the axes of rotation to the eye. The thin lines indicate axes of rotation, the 

 thick the position of muscular attachment. 



line. The plane which passes through the two visual lines is called 

 the visual plane. 



The various positions of the eyeballs are designated primary, 

 secondary, and tertiary. 



The primary position occurs when both eyes are parallel, the 

 visual lines being horizontal (as in looking at the horizon). 



Secondary positions are of two kinds : 



(1) The visual lines are parallel, but directed either upwards or 

 downwards from the horizontal (as in looking at the sky). 



(2) The visual lines are horizontal, but converge towards one 

 another (as in looking at a small object near to and on the same 

 level as the eyes). 



Tertiary positions are those in which the visual lines are not 

 horizontal, and converge towards one another (as in looking at the 

 tip of the nose). 



