124 



THE RODS AND CONES. 



It will be observed that the nerve does not, as one 

 inigbt naturally have expected, enter the eye and then 

 spread itself out at the back of the retina ; but, on the 



contrary, pierces tlie retina 

 and spreads itself out on the 

 front, so that the cones and 

 rods look inwards, and not 

 outwards — towards the back 

 of the eye, and not at the 

 object itself. In fact, we do 

 not look outwards at the 

 actual object, but we see the 

 object as reflected from the 

 base of our own eye. 



From the arrangement of 

 the rods in the eyes of verte- 

 brata, then, the light has 

 necessarily to pass through 

 the retina, and is then re- 

 flected back on it. This 

 involves some loss of light; 

 on the other hand, it perhaps 

 secures the advantage that 

 the sensitive terminations of 

 the rods and cones can be 

 more readily supplied with 

 blood. 



I do not propose to enter 

 into the reason for this 

 peculiar arrangement, which 

 is connected with the development of the eye. Bat 

 it is so different from what might have been expected, 

 is in itself so interesting, and makes so important a 



Fig. 79.— J, Inner segments of rods 

 (s, s, s) and cones (z, z') from man, 

 the latter in connection with the 

 cone- granules and fibres as far as 

 the external molecular layer, 6. In 

 the interior of the inner segment of 

 both rod and cone fibrillar structure 

 is visible. X 800. 



