THE SENSES 951 



The Movements of the Eyes. That the eyes may be efficient 

 instruments of vision, it is necessary that they should have the 

 power of moving independently of the head. An eye which 

 could not move, though certainly better than an eye which could 

 not see, would yet be as imperfect after its kind as a ship which 

 could run before the wind, but could not tack. The mere fact 

 that the angle between the visual axes must be adapted to the 

 distance of the object looked at renders this obvious ; and the 

 beauty of the intrinsic mechanism of the eyeball has its fitting 

 complement in the precision, delicacy, and range of movement 

 conferred upon it by its extrinsic muscles. 



Not only are movements of convergence and divergence of 

 the eyeballs necessary in accommodating for objects at different 

 distances, but without compensatory movements of the eyes 

 it would be impossible to avoid diplopia with every movement 

 of the head ; for the images of an object fixed in one position of 

 the head would not continue to fall on corresponding points of 

 the retinae in another position. 



All the complicated movements of the eyeball may be looked 

 upon as rotations round axes passing through a single point, 

 which to a near approximation always remains fixed, and is 

 situated about 1-77 mm. behind the centre of the eye. 



The position which the eyeballs take up when the gaze is directed 

 to the horizon, or to any distant point at the level of the eyes, is 

 called the primary position. Here the visual axes are parallel, and 

 the plane passing through them horizontal. While the head remains 

 fixed in this position, the eyeballs can rotate up or down around a 

 horizontal axis, or from side to side around a vertical axis ; or 

 upwards and inwards, downwards and outwards, downwards and 

 inwards, and upwards and outwards around oblique axes, which 

 always lie in the same plane as the vertical and horizontal axes of 

 rotation i.e., in the vertical plane passing through the fixed centre 

 of rotation. These facts, spoken of collectively as Listing's law, 

 and first deduced by him from theoretical considerations, were 

 afterwards proved experimentally by Helmholtz and Bonders. It 

 necessarily follows from Listing's law (and this is, indeed, another 

 way of stating it) that in moving from the primary position into any 

 other, there is no rotation of the eyeball round the visual axis no 

 wheel-movement, as it is called. 



A true rotation of the eye round the visual axis does, however, 

 occur when the eyes are converged as in accommodation for a near 

 object, each eyeball rotating towards the temporal side. This is 

 especially the case when the eyes are at the same time converged 

 and directed downwards ; and the rotation may amount to as much 

 as 5. When the head is rolled from side to side, while the eyes 

 are kept fixed on an object, a slight compensatory rotation of the 

 eyeballs takes place against the direction of rotation of the head. 

 The amount of rotation of the eyes is relatively greater for small 

 than for large movements of the head (eye 5 for head 20 ; eye 

 10 for head 8o Kiister). 



