BOnS ANT) CONES 285 



of accoinniodatioii tlirouj^ih loss ol" elasticity of the lens as age 

 advances, the retina retaining its normal position relative to 

 the lens. 



The Retina. ( 1 ) The Mechanism. 



This, as we have seen, is the end-organ for vision. Our aim is 

 now to endeavour to find out how light falling on this surface 

 becomes an effective stimulus for the occipital cortex so that very 

 slight dilTerences in wave-length can be appreciated. There can 

 l)e no doubt that the rods and cones are the actual irritable organs. 

 Once they are stimulated, the disturbance is passed through their 

 cell bodies to their processes in the outer molecular layer where the 

 impulse passes across a synapse to bipolar neurones. These hand 

 on the disturbance to the large oval ganglion cells whose axones 

 form the stratum opticum. The axones are given a myelin 

 sheath as they pass out of th^ eyeball and form the optic nerve. 



Rods and Cones. As both rods and cones make connection 

 in this way with the optic nerve, we have to consider these two 

 types of receiving units. They differ in structure, in distribution, 

 in their associations in the outer molecular layer and in function. 



Structure. Briefly the rods, as their name implies, are columnar 

 structures lying in contact with and normal to the pigmented 

 layer of cells. Their inner portion is attached by a process to a 

 small bipolar cyton whose other process gives off dendrites in the 

 fifth layer (p. 277), several rods forming synaptic connection with 

 one bipolar neurone of the inner nuclear layer. 



Distribution. Rods are not found at the fovea and are associated 

 with the visual purple. 



Function. As more than one rod forms nervous connection 

 with a single bipolar cell, their function cannot be that of exact 

 minute definition. Further, as rods are not found at the fovea, 

 the area of direct vision, they are not necessary for clear vision. 

 Experiment shows that they are sensitive to light of low intensity, 

 i.e. they are effective in twilight vision. When lighting is poor we 

 readily detect movements. The rods, therefore, are easily stinni- 

 lated by light of short duration provided it is not too bright. 

 In the twilight everything appears dressed in shades of grey. Some 

 greys are more easily seen than others. Examination of these greys 

 in a good light shows that they are actually yellows and yellow- 

 greens. From this we infer that the rods are colour blind, but have 

 a lower threshold for light belonging about the middle of tlie 

 visible spectrum. 



Cones are shorter elements and are directly incorporated with 

 their cyton, i.e. they are part of the first order neurones. They 



