SENSE ORGANS OF 
pedance-matching transformer incor- 
porating automatic volume control. 
The organs of sight 
The eyes of birds are progressively 
conservative. They represent the cul- 
mination of a process of development 
which is traceable back to the earliest 
land-living animals, and which has 
been continuously in one direction, 
toward the perfection of daylight 
vision. Human eyes are the product 
of two revolutions, for man’s earliest 
mammalian ancestors, unlike their 
reptilian forebears, were small and 
and only recently, 
nocturnal, by 
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differences between human and avian 
vision are traceable to this history, 
while most of the similarities reflect 
the similar requirements of the present 
day. 
Leaving aside for the moment the 
advantages of mobility and binocu- 
larity, the bird’s eye, considered 
simply as a camera, is the better in- 
strument. It can be seen from figure 
2 that there is less optically wasted 
space, and that the aperture (the ratio 
of maximum pupil diameter to focal 
length) is greater. Moreover the ret- 
ina is shaped so as to lie almost wholly 
in the image plane, so that all distant 
BIRDS—PUMPHREY 
Ficure 1.—Schematic horizontal sections of right half of the heads of man and sparrow to 
show the proportions of eyeball and orbit. Scale chosen to make the eyeballs of equal 
depth. 
geological standards, has his family 
come out again into the daylight. In 
the nocturnal epoch large eyes no 
longer had the same survival value, 
since resolving power had to be sacri- 
ficed to sensitivity, which is nearly 
independent of the size of the eye; and 
man’s family emerged from it with 
eyes loose in their orbits and small in 
proportion to the skull, but forward- 
looking and very mobile (fig. 1). 
There has been no nocturnal epoch 
in the evolution of the birds, and 
evolutionary pressure has been con- 
sistently toward large eyes. The skull 
is built round the orbits, which fill the 
width of it completely, and the eye- 
balls fit the orbits so tightly that their 
freedom of movement is slight or (as in 
the owls) nil. Most of the important 
objects within the visual angle are 
sharply focused on the photosensitive 
cells, whereas in the human eye this 
is only true of objects lying close to 
the optic axis. 
In birds, moreover, the iris and the 
accommodatory mechanism—the stop 
and focusing adjustment—are actuat- 
ed by striated “voluntary”? muscles. 
The operation of accommodation is 
shown diagrammatically in figure 3, 
a. The action of Crampton’s muscle 
is to pull the margin of the cornea in- 
ward, causing a compensatory out- 
ward bulge of the center. Briicke’s 
muscle drags the ciliary body toward 
the axis of the eye, and this in turn 
squeezes the lens, increasing the 
curvature, especially of its external 
surface, 
