SENSE ORGANS OF BIRDS—PUMPHREY 
of movement.’ It is movement to 
which the most primitive eyes respond, 
and sensitivity to movement remains 
in eyes so degenerate that all other at- 
tributes are lost. Unfortunately it is 
an exceedingly difficult function to 
reduce to definition or measurement, 
but some aspects of it are obvious 
enough. Ifastationary field is viewed 
with a stationary eye, a movement of 
a small object in that field will result 
in a change in illumination of a small 
part of the retina, while the excitation 
of the rest of the retina remains con- 
stant. Whether the movement is seen 
as movement or not depends, there- 
fore, not only on the threshold of the 
sensory elements for change and for 
rate of change of illumination, but also 
on the presence of a stationary but 
variegated background. Consequently 
a moving object may fail to be seen as 
moving (a) because it is too small or 
too good a match to the background 
to be seen at all, (b) because it is 
moving too slowly or too fast, or (c) 
because the background, or part of it, 
is also moving, or (d) because the back- 
ground has no landmarks. All these 
conditions are quite obvious and fa- 
miliar, but the background difficulty is 
much less acute for man, most of whose 
activities require him to look horizon- 
tally, than it is for birds, which often 
have to look up at a clear sky or down 
at uniform pasture and moor and sea. 
In such cases there is nothing to hold 
the eye so that the retina can settle 
down to a steady state in which the 
changing excitation due to movement 
of an object will be salient. Yet there 
is abundant evidence that birds as a 
whole are immensely more successful 
than man at detecting movement. 
7 Movement here refers to movement across 
the line of sight. Perception of movement 
along the line of sight is a very much less gen- 
eral accomplishment. To avoid ambiguity, 
it may be necessary to point out this section 
is concerned with initial detection of move- 
ment. The case is distinct from the percep- 
tion of movement in an object which has 
already been fixated by the central fovea, a 
type of movement perception discussed 
above. 
317 
A tentative but fascinating explana- 
tion of this faculty was offered by 
Menner (1938) as a result of his exam- 
ination of the pecten of a number of 
birds, supplemented by one experi- 
ment. The primitive function of the 
pecten is undoubtedly nutritive. Birds 
do not have, and probably could not 
tolerate without a substantial loss of 
acuity, the network of blood vessels 
which spread over and nourish the 
inner layers of the mammalian retina. 
Instead they have the pecten, a more 
or less conical foliated structure whose 
base covers the “‘blind spot’ at the 
entry of the optic nerve and whose 
apex is directed out toward the pupil. 
The pecten is richly supplied with 
blood vessels, and it is presumed that 
nutritive substances diffuse through 
the vitreous humour between it and 
the retina. But though a number of 
fantastic suggestions have been made 
about possible subsidiary functions, no 
one before Menner seriously thought 
of it as anything but a nuisance in the 
dioptric system. Menner, however, 
proved that not only do the foliations 
throw shadows onto a functional part 
of the retina but that the extent of 
foliation, and consequently of the 
shadows, is directly related to habits. 
They are most extensive in hawks; 
diurnal insectivorous birds come next, 
then grain feeders, with nocturnal 
birds easily last (fig. 7). Menner in- 
ferred that the pecten played some 
part in the detection of movement. 
He directed a camera at the sky in 
which some high-flying swifts were 
circling, and, inspecting the image of 
the sky on the ground-glass focusing 
screen, he was unable to detect any 
trace of the images of the moving 
swifts. When a model of a hawk’s 
pecten was stuck on the inner side of 
the ground glass, he found he could 
then easily detect the images of birds. 
The principle of this remarkable 
observation has been confirmed and 
extended by Crozier and Wolf (1943) 
and there is no doubt of its reality, 
but the theory is still very incomplete. 
It is known, however, from other 
