Our sense organs 45

the inside of the eyeball. It contains the photoreceptor
(light-sensitive) cells and four types of
nerve cells, as well as structural cells and epithelial
pigment cells (Latin: pigmentum = dye; Greek:
epithel = outer layer of the skin). The two kinds
of photo-cells are called rods and cones because
of their shape. These microscopically small light
detectors, which contain the various visual pigments,
are masterpieces of technological efficiency.
Each eye has about 110 million rods and 6
million cones. They form a laterally interconnected
network and are connected “vertically”,
by means of so-called bipolar cells, to the one
million ganglion cells. These collect all the optical
signals received by the retina, determine the
direction of flow of these signals, and transmit
them to the brain through the optic nerve. This
bundle of more than one million nerve fibres,
each well “insulated” from the others, is about
2 mm thick. Present-day communications experts
using glass fibre technology can only dream of
a “cable” of this kind.
One single square millimetre of the retina contains
approximately 400,000 optical sensors. To get
some idea of such a large number, imagine a
sphere, on the surface of which circles are drawn,
the size of tennis balls. These circles are separated
from each other by the same distance as their
diameter. In order to accommodate 400,000 such
circles, the sphere must have a diameter of 52
metres, nearly three times as large as the hot air
balloons used for advertising promotions.
The photo-receptors: The rods and cones not
only differ in shape, but also in function. The
rods are cylindrical, while the cones are smaller
and have a tapered form. In the case of low illumination
as at night, the rods enable us to dis-
tinguish between brightness and darkness. They
are so sensitive that the absorption of a single
photon results in a measurable electrical signal.
This high sensitivity is achieved through having
a long time lag (about 0.3 seconds) between the
absorption of a photon and the emission of the
electric signal, allowing a complex amplification
process to take place.
The cones operate much faster; their time lag is
only 0.075 seconds, but they are much less sensitive
than the rods, and only function optimally in
daylight. There are three types of cones, distinguished
by their absorption maxima, each being
most sensitive for, respectively, red light (having
a wave length of approximately 705 nm), green
light (520 nm), and blue light (450 nm). By comparing
the messages received from the different
cones, the ganglia identify the colours actually
observed.
We would expect the light receptors to be on the
side of the retina exposed to the incident light,
but, amazingly, this is not the case. The light
must first pass through another layer of the retina.
That is why it has been said that our eyes
have “inverted wiring”, an arrangement which
nevertheless works brilliantly.
The light sensitive cells act like interpreters,
translating the impulses of light into the language
of the nervous system. Another way of
putting it is that a photo-receptor cell is basically
a counter which counts the number of incident
light quanta (photons). Its sensitivity ranges over
five powers of ten, and it is able to adapt to the
brightness of the prevailing light conditions by
altering its sensitivity. For example, in response
to bright light, it can reduce its sensitivity
100,000 times!
Sensitivity: We are blessed with extremely sensitive
sense organs. Furthermore, the Creator
solved a universal technical problem. Whenever
a radio receiver is set for maximum sensitivity, it
becomes noisy. This hissing sound is caused by
the irregular thermal (heat) motions of electrons
in the resistors. It can be eliminated by cooling all
the components to a temperature far below
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