290 OUTPOSTS OF THE INTELLIGENCE SERVICE 



blurred, then remains in this blurred state apparently without 

 change for a few seconds before involuntary movements of the 

 body as a whole (respiratory or cardiac) cause a slight improvement 

 in vision j^ro tern. The blurriness is primarily due to points of 

 the image on the fovea alternating between adjacent cones and so 

 allowing one cone to have its irritability restored while its neighbour 

 is being excited. This rhythmic alternation of ability to function 

 is a universal characteristic of biological systems and permits of 

 a function being effectively carried out almost continuously over a 

 reasonable period of time by any collection of units, e.g. cones, 

 capillaries to an area, touch receptors, etc. 



Movements of the Eyeball. 



The eyeball lies at the front of the bony orbit, a cone-shaped 

 ^anal with its apex directed backwards and pierced by the optic 



TRANSVEKSE 



Fig. 74. — Diagram of extrinsic muscles of eye. 



foramen. The antero-posterior or visual axis of the eye, i.e. the 

 line passing through the centre of the cornea and the fovea makes 

 an angle of about 20° with the long axis of the orbit. The move- 



