228 HAND-BOOK OF PHYSIOLOGY. 



antagonistic condition of that membrane; and the opposite conditions of 

 which the retina thus becomes the subject would seem to balance each 

 other by their reciprocal reaction. A necessary condition for the pro- 

 duction of the contrasted colors is, that the part of the retina in which 

 the new color is to be excited, shall be in a state of comparative repose; 

 hence the small object itself must be grey. A second condition is, that 

 the color of the surrounding surface shall be very bright, that is, it shall 

 contain much white light. 



Movements of the Eye. The eyeball possesses movement around 

 three axes indicated in Fig. 385, viz., an antero-posterior, a vertical, and 

 a transverse, passing through a centre of rotation a little behind the 

 centre of the optic axis. The movements are accomplished by pairs of 

 muscles. 



Movements. By what muscles accomplished. 



Inward ..... Internal rectus. 



Outward . . . . External rectus. 



TT -, j Superior rectus. 



U P ward j Inferior oblique. 



Downward . Inferior rectus. 



Inward and upward 



Inward and downward . Internal and inferior rectu8 ' 



Outward and upward . 

 Outward and downward 



Superior oblique. 



Internal and superior rectus. 



Inferior oblique. 



Superior oblique. 



External and superior rectus. 



Inferior oblique. 



{ Jj*ri -tus. 



OF THE SIMULTANEOUS ACTION OF THE Two EYES. 



Although the sense of sight is exercised by two organs, yet the im- 

 pression of an object conveyed to the mind is single. Various theories 

 have been advanced to account for this phenomenon. By Gall it was 

 supposed that we do not really employ both eyes simultaneously in vision, 

 but always see with only one at a time. This especial employment of 

 one eye in vision certainly occurs in persons whose eyes are of very un- 

 equal focal distance, but in the majority of individuals both eyes are simul- 

 taneously in action, in the perception of the same object; this is shown 

 by the double images seen under certain conditions. If two fingers be held 

 up before the eyes, one in front of the other, and vision be directed to 

 the more distant, so that it is seen singly, the nearer will appear double; 

 while, if the nearer one be regarded, the most distant will be seen double; 

 and one of the double images in each case will be found to belong to one 

 eye, the other to the other eye. 



