Biology Reference
In-Depth Information
Fig. 5.1
causes the pupil to look black. Notice that the light-sensitive cells receive light only
after it has traversed the covering layer of nerve cells that send impulses generated
by the light to the brain via the optic nerve. This does not seem to be the opti-
mum position for the light-sensitive cells, so the human eye may not be as perfect
as Darwin thought - an engineer designing the most efficient camera puts the film
directly in the path of the light. By contrast, the eyes of molluscs such as the octopus
have a better design - in these species, the light-sensitive cells form the top layer of
the retina.
There are two types of light-sensitive cells, called rods and cones after their
shapes. Rods and cones are nerve cells specialised to absorb photons of light, whose
absorption triggers electrical impulses that are relayed by synapses to neighbouring
nerve cells, and so onto the brain. Rods detect light intensity only, and so enable
black and white images to be formed in the brain, but the cones in addition detect
different wavelengths of light and so enable colour vision. The rods are more sen-
sitive to light than the cones, which is why colours are so difficult to see in dim
lighting. But the cones can resolve more detail than the rods. The reason for this
difference in resolving power between rods and cones is that only one cone cell is
connected to the next nerve cell in the relay, but several rod cells are connected to
the next nerve relay cell (see diagram of the retina in Figure 5.1). There are about
one hundred million rod cells in the human eye, but only about three million cone
