834 A MANUAL OF PHYSIOLOGY 



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 retinas 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 

 lacts, spoken of collectively as Listing's law, and first deduced by 

 him from theoretical considerations, were afterwards proved experi- 

 mentally 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). 



The Extrinsic Muscles of the Eye. The eyeball is acted 

 upon by six muscles arranged in three pairs, which may be 

 considered, roughly speaking, as antagonistic sets. These 

 are the internal and external recti, the superior and inferior 

 recti, and the superior and inferior obliqui. 



