548 PHYSIOLOGY 



A picture formed by the bleaching of the visual purple in those 

 parts of the retina which correspond to the high lights of the image 

 formed by the lens, can be fixed, much like a photograph, by immersing 

 the retina in a solution of alum. Fucsin is the pigment found in the 

 form of needles, plates or prisms in the processes of the cells of the stratum 

 pigmenti (the outer layer of the retina). The object of this pigment is 

 apparently to absorb light which might tend "to spread from those retinal 

 elements, on which an image of a light source is falling, to neighbouring ones. 

 Some of this pigment is bleached by strong light, but so far as is known 

 the break-down products have no visual function to perform. The presence 

 of other pigments has been described in the retina, such as visual yellow and 

 the bright pigment granules found in birds. Their presence is too variable 

 for them to be considered to take any essential part in the visual mechanism. 



PHYSIOLOGICAL CHANGES produced by light depend greatly on 

 the region of the retina on which they fall, since this may contain rods 

 only, cones only, both rods and cones, or neither rods nor cones. The 

 peripheral parts of the retina contain numerous rods and very few cones. 

 When stimulated by light of low intensity, this part of the retina is found 

 to be exceedingly sensitive, particularly if the eyes have been closed or 

 kept in the dark for a time. Tests with light of low intensity and of different 

 colour shows that the region is colour-blind, but that rays in the middle of 

 the spectrum are more readily appreciated than others. We have here 

 well developed the so-called twilight vision, which is associated with the 

 rod- visual purple mechanism just described. Besides being very sensitive 

 to light of low intensity, the periphery of the eye is particularly perceptive 

 of light of low intensity and short duration. This part of the retina there- 

 fore appreciates movement at night very readily. Lastly, owing to the fact 

 that a number of rods connect with one nerve fibre which conveys the 

 impulses to the brain, the periphery of the eye has a poor perception of 

 detail. 



THE FOVEA CENTRALIS is found to contain cones only. The vision 

 in this region is therefore the antithesis of that found in the periphery. The 

 appreciation of light of low intensity is bad, but when an image is sufficiently 

 bright to cause stimulation, its colour is perceived. When light is 

 poor, rapid motion is not so well observed as it is by the periphery. 

 There is an extraordinary acuteness at perceiving fine detail. This is due 

 to the fact that the cones in the fovea are very closely packed, so 

 rio. vly that they become flattened \vhere they touch one another 

 and thus have a hexagonal shape in transverse section. Further 

 each cone is connected to its own nerve fibre, so that no cyphering of tin- 

 Impulses can occur on the way to the brain. Experiments on visual acuity 

 definitely show that the fineness of the detail, which the eye can perceive 

 at the foveal region, is fully as great as that which we should expect 

 to find, if each cone acted quite independently of its neighbours. Parts of 

 the retina around the macula lutea, since they contain both rods and 

 cones, possess ;is \\c should expect both the power to perceive colour found 



