BINOCULAR VISION. 369 



eyeS; _that is, that position taken by the eyes when we look straight 

 before us toward the horizon, a position, therefore, in which the 

 plane of the horizontal axes is parallel to the ground; all other 

 positions of the eyes are spoken of as secondary. 



With regard to the movements of the eyes about Its axes of 

 rotation the following general statements are made : Starting from 

 the primary position, rotations of the eyes about the vertical axis 

 that is, movements directly to right or left may be made by the 

 contraction of the internal or the external rectus as the case may be. 

 Rotations around the transverse axis that is, movements directly 

 up or down require in each case the co-operation of two muscles. 

 In movements upward the superior rectus, acting alone, would in 



Fig. 157. " Diagram showing for the left eye the paths of the line of sight caused by the 

 action of the different eye-muscles (Hering). The horizontal line indicates movements out or 

 in to various degrees as caused by the contraction of the internal or external rectus. The 

 curved lines show the amount of torsion given the eyeball by the superior and inferior 

 rectus and the superior and inferior oblique when contracting separately. Tne short 

 heavier line at the end of the paths indicates the position of the horizontal meridian at 

 the end of the movement." R. e., the external rectus; R, i., the internal rectus; R. S., 

 the superior rectus; R. inf., the inferior rectus; O. i., the inferior oblique; O. S. t the superior 

 oblique. 



rotating the eyeball upward also give it a slight torsion so as to 

 turn the upper part of the vertical meridian inward. To obtain a 

 movement directly upward (rotation around the transverse axis) 

 the superior rectus and inferior oblique must act together. For 

 a similar reason rotation directly downward requires the com- 

 bined action of the inferior rectus and superior oblique. These 

 facts are expressed clearly in Hering' s diagram, reproduced in 

 Fig. 157, which indicates the paths traversed by the line of sight 

 when the eyeball is moved by the different muscles acting sepa- 

 rately. Rotation of the eyeballs around oblique axes require 

 the co-operation of three of the muscles : movements upward 

 and outward the superior rectus, inferior oblique, and external 

 rectus; movements upward and inward superior rectus, inferior 

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