THE ORGANS OF SENSE. 137 



a green, blue on a yellow, and yellow on a blue, the grey strip in each 

 case assuming the colour complementary to that of the background. 

 This phenomenon is supposed by Hering to be due to dissimilation of 

 any one of the three photo-chemical substances in part of the retina, 

 this being accompanied by assimilatory changes in the same substance 

 in the adjacent parts of the retina (or the converse), a process known 

 as "retinal induction." 



Successive contrast is seen if one gazes at a coloured disc, for 

 example red, for half a minute or a minute, and then transfers the gaze 

 to a white sheet. The complementary colour will be seen on the sheet : 

 it is green if the original disc be red, blue if it be yellow, and so on, and 

 it constitutes one form of negative after-image. On Hering's theory the 

 after-image is due to anabolic changes following katabolic, or vice versa. 

 On another theory, however, the phenomenon is ascribed to changes in 

 the cortex of the cerebral hemisphere. 



THE MOVEMENTS OF THE EYEBALL. 



In a state of rest the two eyeballs lie in the orbital cavities with 

 their optic axes projecting horizontally forwards and parallel with each 

 other. Conjugate movements of the two eyes take place, either 

 upwards or downwards, or to the right or left ; or certain of these 

 movements may be combined or may be accompanied by rotation of 

 the eyeballs. Further, during accommodation there is a movement 

 of convergence of the eyeballs. 



These various movements take place about the three principal 

 axes of each eyeball, the antero-posterior, vertical, and horizontal 

 axes, and are effected by the six extra-ocular muscles of each eye. 

 The cornea is rotated upwards and inwards by the superior rectus, 

 upwards and outwards by the inferior oblique, and directly upwards 

 by the combined action of these two muscles. The inferior rectus 

 rotates the cornea downwards and inwards, the superior oblique turns 

 it downwards and outwards, and both these muscles together turn it 

 directly downwards; the internal rectus rotates the eyeball inwards, 

 the external rectus turns it outwards. The cornea may be rotated 

 into intermediate positions by the combined action of two or more 

 muscles acting together. When any one muscle contracts there is 

 reciprocal relaxation of its antagonist ; thus contraction of the internal 

 rectus is accompanied by relaxation of the external rectus of the 

 same eye. 



The external rectus is supplied by the sixth, the superior oblique 

 by the fourth, and the remaining muscles, together s with the levator 

 palpebrse superioris, by the third cerebral nerve. 



