1864.] 



On the Normcd Motions of the Human Eye. 



187 



that the superior oblique muscle lowers the visual line, and the inferior raises 

 it ; but these last two muscles not only raise and lower the visual line ; 

 they produce also a rotation of the eye round the visual line itself, of which 

 we shall have to speak more afterwards. 



A solid body, the centre of which is fixed, and which can be turned 

 round three different axes of rotation, can be brought into every possible 

 position consistent with the immobility of its centre. Look, for instance, at 

 the motions of our arm, which are provided for at the shoulder-joint by the 

 gliding of the very accurately spherical upper extremity of the humerus in 

 the corresponding excavation of the scapula. When we stretch out the arm 

 horizontally, we can turn it, first, round a perpendicular axis, moving it 

 forwards and backwards ; we can turn it, secondly, round a horizontal axis, 

 raising it and lowering it ; and lastly, after having brought it by such 

 motions into any direction we like, we can turn it round its own longitu- 

 dinal axis, which goes from the shoulder to the hand ; so that even when 

 the place of the hand in space is fixed, there are still certain dijfferent posi- 

 tions in which the arm can be turned. 



Now let us see how far the motions of the eye can be compared to those 

 of our arm. We can raise and lower the visual line, we can turn it to the 

 left and to the right, we can bring it into every possible direction, through- 

 out a certain range — as far, at least, as the connexions of the eyeball permit. 

 So far the motions of the eye are as free as those of the arm. But when 

 we have chosen any determinate direction of the eye, can we turn the eye 

 round the visual line as an axis, as we can turn the arm round its longi- 

 tudinal axis ? 



This is a question the answer to which is connected with a curious 

 peculiarity of our voluntary motions. In a purely mechanical sense, we 

 must answer this question in the affirmative. Yes, there exist muscles 

 by the action of which those rotations round the visual line can be per- 

 formed. But when we ask, "Can we do it by an act of our will?" we must 

 answer, " No." We can voluntarily turn the visual line into every possible 

 direction, but we cannot voluntarily use the muscles of our eye in such a 

 way as to turn it round the visual line. Whenever the direction of the 

 visual line is fixed, the position of our eye, as far as it depends upon our 

 will, is completely fixed and cannot be altered. 



This law was first satisfactorily proved by Professor Bonders, of Utrecht, 

 who, in a very ingenious way, controlled the position of the eye by those 

 ocular spectra which remain in the field of vision after the eye had been 

 fixed steadily daring some time upon any brightly coloured object. I have 

 used for this purpose a diagram like fig. 1 : the ground is grey paper, and 

 in the middle, along the line a b, is placed a narrow strip of red paper on 

 a broader strip of green paper*. The centre of the red strip is marked 

 by two black points. When you look for about a minute steadily and 

 without moving your eye at the centre of the diagram, the image of the 

 * Green is represented in the figure by white ; red by the central dark stripe. 



