VISION 



1023 



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 Donders. It necessarily follows from Listing's law "(and this is, 

 indeed, another way of stating it) that in moving from the primary posi- 

 tion 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 down- 

 wards; 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 80 Kiister). 



The Extrinsic Muscles of the Eyes. 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. 



Although the movements of the 

 eye have been very fully studied, 

 and are, upon the whole, well 

 understood, our knowledge of the 

 manner in which any given move- 

 ment is brought about, and of the 

 exact action of the muscles which 

 take part in it, is by no means ft ext 



as copious and precise. From Fig. 433. Horizontal Section of Left 



the nature of the case, the greater E Y e - Arrows show direction of 

 part of what we do know has pu " of th / t , mu9c f les - The axis of 



, rotation of the external and internal 



been inferred irom the anatomical recti would pass through the inter- 

 relations of the muscles as re- section of a and /3 at right angles 

 vealed by dissection in the dead ' the plane of the paper - 



body rather than gained from actual observation of the living eye. 

 A plane, called the plane of traction, is supposed to pass through the 

 middle points of the origin and insertion of the muscle whose action 

 is to be investigated, and through the centre of rotation of the 

 eyeball. A straight line drawn at right angles to this plane through 

 the centre of rotation is evidently the axis roand which the muscle 

 when it contracts will cause the eye to rotate, provided that th3 

 fibres of the muscle are symmetrically distributed on each side of 

 the plane of traction. The axes of rotation of the antagonistic 

 pairs almost, but not completely, coincide with each other. The 

 common axis of the external and internal recti practically coincides 

 with the vertical axis of the eyeball (Fig. 433) in the primary posi- 



