2 7 8 



HUMAN PHYSIOLOGY 



50-60, upward 45-55, downward 65-70. The perimeter 

 is used in determining the field. 



II. Movements of the eyes. 



I. Movement of one eye. (a) General observations. 

 The external eye muscles turn the eyeball about a point 

 lying on the optical axis, 13.557 mm behind the cornea. 

 The eye and its socket form a ball-and-socket joint (see 

 page 202). 



The monocular field of rotation [Blickfeld] is the field 

 which includes all the luminous points that can be seen by 

 the moving eye, the head being held quiet. 



Primary position is the position of the eye when a person 

 looks straight forward into the distance, the head being held 



FIG. 35. MUSCLE AND AXES OF ROTATION OF THE EYE. (After Helmholtz.) 



a, rectus externus; s, rectus superior; i. rectus internus; t, obliquus superior; 

 u, trochlea; A, optical axis; DD, axis of rotation of the rectus superior and 

 inferior; B, axis of rotation of the obliquus superior and inferior; v y insertion of 

 obliquus inferior. 



erect. In the primary position the visual axis is horizontal 

 and parallel to the median plane of the body. For any 

 given position of the visual line it is mechanically possible 

 that the eye can still assume many positions because of its 



