836 A MANUAL OF PHYSIOLOGY 



axis of the superior and inferior recti, @, lies in the horizontal 

 visual plane in the primary position, but makes an angle of 

 about 20 with the transverse axis, its inner end being tilted 

 forwards. The consequence is that contraction of the 

 superior rectus turns the eye up, and contraction of the 

 inferior rectus turns it down, but both movements are also 

 combined with a slight inward rotation. The common axis 

 of the oblique muscles, a, makes an angle of 60 with the 

 transverse axis, the outer end of it being the most anterior. 

 The direction of traction of the superior oblique is, of course, 

 given not by the line joining its bony origin and its insertion, 

 but by the direction of the portion reflected over the pulley. 

 When the superior oblique contracts alone, the eyeball is 

 rotated outwards and downwards; the inferior oblique 

 causes an outward and upward rotation. None of the 

 common axes of rotation of the pairs of muscles, except 

 that of the external and internal recti, lies in Listing's plane. 

 Now, as we have seen that every movement which the eye, 

 supposed to be ojiginally in the primary position, can 

 execute may be considered as a rotation round an axis in 

 this plane, it is clear that every movement, except truly 

 transverse rotation, must be brought about by more than 

 one pair of muscles. For vertical rotation, the inward pull 

 of the superior rectus is antagonized by a simultaneous out- 

 ward pull of the inferior oblique ; for downward rotation, 

 the inferior rectus and superior oblique act together. In 

 oblique movements, a muscle of each of the three pairs is 

 concerned. The effect on the eyeball of simultaneous con- 

 traction of certain pairs of muscles may be summarized thus: 



External rectus (outward) + internal rectus (inward) *= none. 



Superior rectus (upward and inward) + inferior oblique (upward 

 and outward) = upward 



Inferior rectus (downward and inward) + superior oblique (down- 

 ward and outward) = downward. 



