394 



PHYSIOLOGY 



CHAP. 



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

 position l>y 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 ami 

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



Fn:. 184. Dia.^idiii >huuit,_: tin- direction of Mum-input when tin- i-yi-ball is rotated from the 

 primary iiMtioii by the ai-tiun nt' tin- diU'en-nt musrl.-s. (H'-ring.) The angles of rotation 

 corresponding ID the IIID\ emenls of I hi- visual axes are indicated on the line-, in decrees, dl 

 ie]ire>Hnt- lip- distance of the plane of sight (which here corresponds with the plane of the 

 li^nrp) from the centre of rotation of the eye. The position of the horizontal meridian at the 

 end of the movement is indicated 1>\ the short heavy line at tin- extreme . 



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 : - 



