THE OCULAR MUSCLES. 



785 



On looking upicards in the tertiary position, the upper ends of the vertical lines of separation 

 of the retina diverge ; on looking downwards they converge. If the visual plane be raised, the 

 eye, when it deviates laterally to the right, makes a circular rotation to the left. When the 

 visual plane is depressed, on deviating the eye to the right or left, there is a corresponding 

 circular rotation to the right or left. Or we may express the result thus : When the angle of 

 elevation and the angle of deviation have the same sign ( + or - ), then the rotation of the eye- 

 ball is negative ; when, however, the signs are unequal, the rotation is positive. In order to 

 make the circular rotation visible in one's own eye, accommodate one eye for a surface divided 

 by vertical and horizontal lines until a positive after-image is produced, and then rapidly rotate 

 the eye into the third position. The lines of the after-image then form angles with the lines 

 of the background. As the position of the vertical meridian of the eye is important from a 

 practical point of view, it is necessary to note that, in the primary and secondary positions of 

 the eyes, the vertical meridian retains its vertical position. On looking to the left and up- 



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Fig. 566. 

 Scheme of the action of the ocular muscles. 



wards, or to the right and downwards, the vertical meridians of both eyes are turned to the 

 left ; conversely, they are turned to the right on looking to the left and downwards, or to the 

 right and upwards. 



In the secondary positions of the eye, rotation of the axis of the eye never occurs (Listing). 

 Very slight rolling of the eyes occurs, however, when the head is inclined towards the shoulder, 

 and in the direction opposite to that of the head, it is about 1 for every 10 of inclination of 

 the head (SJcrehitek). 



Ocular Muscles. The movements of the eyeball are accomplished by means of 

 the four straight and two oblique ocular muscles. In order to understand the 

 action of each of these muscles, we must know the plane of traction of the muscles 



3d 



