Biology Reference
In-Depth Information
to structure and occurrence. Thus, he pointed to several characteristics
that may be summarized as follows (Schultze,
1866
, pp. 247-251):
1.
Rods are generally longer than cones, and the rod and cone outer segments
have rod- and cone-like shapes, respectively, as the names indicate.
2.
The terminal of the inner segment of the cones is much thicker than that
of the rods and divides into several thin fibres.
3.
Cones of many diurnal bird species possess yellow, red or colourless
strongly light-refracting droplets (globules) between their inner and
outer segments, positioned so that light has to pass through them before
reaching the outer segment.
4.
Several diurnal (lizard, snake, etc.) and nocturnal (bat, mole, shark,
hedgehog, mouse, skate, etc.) species have only cone and rod retinas,
respectively.
5.
The central fovea of humans, apes and falcons contains only cones.
Based on points 4 and 5, Schultze (
1866
) concluded that cones and
rods mediated day and night vision, respectively, and that both receptor
systems functioned as light and space receptors. In order to decide
whether both receptor systems might also mediate chromatic sensation,
Schultze (
1866
) compared day and night vision in humans. He referred
to the extensive study of Aubert (
1865
) who had found that colour vision
changed qualitatively during dark adaptation with a gradual transition
from chromatic to achromatic vision where, eventually, only brightness
differences were observed. Consequently, Schultze (
1866
) concluded
that only the cone receptor system could mediate chromatic vision.
In accordance with this conclusion, he pointed to the fact that colour
vision deteriorated when a test field was moved outward from the
rod-free fovea, while the number of rod receptors activated increased.
3.3 One or several types of cone?
Having presented strong arguments in favour of the assumption that only
cones may mediate chromatic colour vision, Schultze (
1866
) sought to
develop his theory a step further by attempting to answer the fundamental
question of whether there were one or several types of cone in the retina.
In his investigation of the retinas of mammals and fish he
could find no anatomical evidence for different types of cone
