378 THE BODY AT WORK 



cone has its proper ganglion-cell, collecting cell, and efferent 

 fibre. Rods are served in groups by ganglion-cells and col- 

 lecting cells. From this it may be inferred that a cone is a 

 sensory unit, an inference confirmed, as we shall show presently, 

 by direct evidence. The connections of the rods show that 

 they also are sensory elements, although it may be doubted 

 whether they are sensory units. The optic nerve contains a very 

 large number of fibres about a million all small, but some 

 distinctly larger than the rest. The largest very probably belong 

 to the collecting cells of rods. But the retina certainly does 

 not contain a million collecting cells. A considerable residue 

 of fibres is therefore unaccounted for. It is supposed that 

 they are afferent to the retina, but we have no knowledge 

 regarding the nature of the impulses which descend from the 

 brain. 



The retinal pigment is not merely a backing for the sensitive 

 screen. It undoubtedly plays an important part in vision. 

 That it is not essential is evident from the fact that albinos, 

 whose eyes appear pink owing to the absence of pigment, and 

 the consequent showing through of the blood in the exceedingly 

 vascular membrane which lies behind the retina, can see ; 

 although their visual sense cannot be described as normal. 

 They are exceptionally sensitive to an excess of light. We 

 shall return to this subject after describing the differences in 

 manner of functioning which distinguish rods from cones, 

 differences so marked as to justify us in speaking of two kinds 

 of vision. 



During twilight warm tones gradually fade out of the 

 landscape ; cold blues and greys predominate. A time arrives 

 when scarlet poppies look black, although yellow and blue 

 flowers and green leaves can still be dimly distinguished. In 

 full daylight colours are seen at their brightest in the high 

 lights ; where the light is dim they tend to appear in different 

 shades of grey. At night, if the sky is star-lit, all colours give 

 place to a slightly bluish grey in the high lights, black in the 

 shade. But a not very uncommon abnormality is night- 

 blindness inability to see at all when the light is not bright 

 enough for the recognition of colours. In persons so affected 

 the rods do not function ; for it is with the rods that we see in 

 weak light. They record differences in intensity between the 



