280 HUMAN PHYSIOLOGY 



cornea, i.e. raising, lowering, adduction (to nasal side) and 

 abduction (to malar side). 



2. Deviation of the perpendicular meridian of the cornea 

 in the primary position from the perpendicular (wheel move- 

 ment inward when the upper part of the meridian is bent 

 toward the median plane ; wheel movement outward when 

 this part is bent away from the median plane). 



The eye is turned from the primary position : 



1. By the rectus externus ; abduction. 



2. By the rectus in ternus; adduction. 



3. By the rectus superior; upward, adduction and wheel 

 movement inward. 



4. By the rectus inferior; downward, adduction and 

 wheel movement outward. 



5. By the obliquus inferior; upward, abduction and wheel 

 movement outward. 



6. By the obliquus superior; downward, abduction and 

 wheel movement inward. 



The action of these muscles is illustrated in Fig. 36. 



(c} Combined action of the muscles of one eye. The rectus 

 superior and obliquus inferior are always simultaneously 

 innervated (from a coordination centre) ; also the rectus 

 inferior and obliquus superior. 



The secondary position, i.e. abduction, adduction, raising, 

 lowering, are not accompanied by wheel movements. By 

 simple raising and lowering, the adduction and wheel move- 

 ment produced by one of the active muscles is destroyed by 

 the opposite action of the other muscle. All other move- 

 ments (tertiary position} are associated with wheel move- 

 ments. This wheel movement takes place: 



1. Outward (p. inf.} by raising (r. sup.} and abduction 

 (r. ext.}; 



2. Inward (r. sup.} by raising (p. inf.} and adduction 

 (r. int.}', 



3. Inward (p. sup.} by lowering (r. inf.} and abduction 

 (r. ext.}\ 



