394 



PHYSIOLOGY 



CHAP. 



The eyes are rotated to the right or left from the primary 

 position by contraction of the external rectus of the right and the 

 internal rectus of the left side, and vice versa. But to move the 

 eyes up or down, the contraction of the two superior or inferior 

 recti is not sufficient, the associated contraction of the oblique 

 muscles being also required that of the superior rectus and 

 inferior oblique in raising the eye, of the inferior rectus and 

 superior oblique in lowering it. These two pairs of muscles work 



FIG. 184. Diagram showing the direction of movement when the eyeball is rotated from the 

 primary position by the action of the different muscles. (Hering.) The angles of rotation 

 corresponding to the movements of the visual axes are indicated on the lines in degrees, dd 

 represents the distance of the plane of sight (which here corresponds with the plane of the 

 figure) from the centre of rotation of the eye. The position of the horizontal meridian at the 

 end of the movement is indicated by the short heavy line at the extreme ends. 



together in turning the eye up or down, but their action is also 

 antagonist since the recti rotate it inwards and the obliqui rotate 

 it outwards ; these opposite rotations, however, compensate each 

 other, so that the resultant of the double action is an upward or 

 downward movement of the eye. 



The synergic action of two muscles does not suffice for 

 the diagonal or oblique movements which carry the eyeball 

 into the so-called tertiary positions, and three muscles are in- 

 volved : 



