544 



PHYSIOLOGY 



those found elsewhere. They arc very closely packed, so that their inner limbs 

 are seen in transverse section to have a hexagonal shape, the flat surfaces being 

 in contact with those -of their neighbours. (3) The rows of nerve cells and 

 dendrites, which in the rest of the retina lie approximately in line with the rod 

 or cone to which they belong, are in the fovea pressed to one side, in a direction away 

 from the centre. In this way a cone may have the nerve cells to which it is connected 

 placed at a considerable distance away in the surrounding macula. It is this displace- 

 ment of the nerve fibres and their cells that causes the fovea to appear hollow. The 



I i<;. 274. Section through half the fovea centralis. 

 and HOLDING BIRD.) 



purpose of this physiological arrangement would appear to be without question the 

 avoidance, at this important region of the retina, of the scattering of the image whirh 

 passage through the nerve cell layers would introduce. (4) The fovea unlike tin- 

 rest of the retina is devoid of blood vessels. The purpose of this arrangement would 

 appear to be similar to that just given. (5) Visual purple is said to be absent from the 

 fovea, This would appear to be connected with the absence of rods. 



l!"imd the fovea is a ring in which rods and cones are present in almost c<|ii;il 

 number. In still more peripheral regions cones are relatively few, and sever.il i<><l> 

 connect \\ith each avon ; this reduces the relative number of nerves. 



CHANGES IN THE RETINA ON EXPOSURE TO LIGHT 



A light stimulus falling on the retina causes a number of changes to occur 

 which may be classed as structural, physical, chemical and physiological. 



STRUCTURAL CHANGES occur on exposure of the eye to light: 

 firstly, movement of the pigment from the outer epithelial layer into 

 the space between the rods and cones, secondly, shortening of the 

 cones themselves. These changes occur only when the connections of the 

 eye with the brain, are intact. The rate of movement appears to vary with 

 intensity, and violet light is said to be better than red. It is interesting to 

 find that electrical stimulation of the optic nerve or the falling of light on 

 the other retina to that of the eye observed also causes these cone move- 



