MOVEMENTS OE THE EYEBALLS 655 



way the inferior rectus muscle will be associated with the superior 

 oblique. 



As we have seen, four out of six muscles when acting singly cause 

 rotation of the eyeball on its antero-posterior axis. The question 

 arises whether these movements of rotation ever occur under normal 

 circumstances. This may be tested in the following way : The gaze 

 is first directed at a brilliant line of light, e.g. a straight electric 

 incandescent filament. The gaze is then directed to a uniform white 

 surface. On this white surface a negative after-image of the vertical 

 line is seen. It is found that in whatever direction the eyes be turned, 

 upwards, downwards, or obliquely to right or left, the after-image 

 always retains its vertical direction. If there were rotation of the 

 eyeballs on their antero-posterior 

 diameters, the stimulated portions 

 of the retina, i.e. those which are 

 the seat of the after-image, would ^ ' 30 



lie obliquely, and the apparent 

 direction of the negative after- ** 

 image would be also oblique. 

 We see therefore that, under 

 normal circumstances, no rota- 

 tion of the eyeball on its 

 antero-posterior diameter takes 

 place. The actions of the dif- FIG. 298. Diagram to show direction in 



ferent muscles are always so ^^^^f^ 

 co-ordinated that all movements 



of the eyeballs take place round axes, which lie in a plane passing 

 through the centre of rotation of the eyeballs and at right angles 

 to the visual axes. 



In man movements of the eyes are always bilateral and take place 

 in such a way that an image of one and the same object will fall on the 

 central spot of each retina. These movements are simple in character 

 and are of only two kinds, viz. : 



(1) Movements of both eyes with maintenance of parallelism of 

 the visual axes ; to this class belong the movements of conjugate 

 deviation employed in following the passage of an object across the 

 field of vision from right to left, or vice versa, as well as less extensive 

 upward and downward movements of both eyeballs. 



(2) The movement of convergence of the axes of both eyes, which 

 is always associated with accommodation for near objects, and there- 

 fore with contraction of the ciliary muscles and of the constrictors of 

 the pupils. 



Other movements can be effected, but only with effort. Thus 

 we can converge the axes of the eyes so as to look at a near object 



