SENSE ORGANS OF 
pretation of the function of the steep- 
sided fovea is correct, it is at least 
consistent with the current view that 
detection of movement is a more 
primitive visual function than inter- 
pretation of static detail, for the pis- 
cine and the reptilian foveae resemble 
the typical avian central fovea much 
more closely than the primate fovea; 
and it also affords a rational explana- 
tion of the extreme steepness of profile 
of the central fovea of kingfishers, 
(fig. 5 6), which is manifestly incom- 
patible with Walls’ theory. 
A central fovea is present in most 
avian retinae with a well-marked cen- 
tral area. A temporal fovea is also 
BIRDS—PUMPHREY 313 
are directed more forwardly than is 
commonly the case for birds, with a 
consequent sacrifice of the ability to 
“see behind,’ which most birds with 
laterally directed eyes retain. It is 
noteworthy that owls, which have an 
even larger binocular field than 
hawks, have eyes which are rigidly 
fixed in their sockets and quite in- 
capable of convergence, but here the 
retina is of the rod-rich nocturnal type 
and the temporeal fovea is vestigial or 
absent. (There is no area or central 
fovea. ) 
This is a convenient place to com- 
ment on one profound difference be- 
tween the vision of birds and man. 
Figure 6.—Diagrammatic horizontal section of bird’s head to show the line of sight of 
temporal and central foveae. 
present in nearly all birds which pur- 
sue moving prey in the daytime— 
hawks, kingfishers, shrikes, swallows, 
terns, etc.—and in the hummingbirds, 
whose feeding habits necessitate a very 
nice adjustment of hovering flight 
(pl. 2, lower left). And in all these 
birds, as has been said, it is so situated 
that the image of an object ahead is 
formed on the temporal foveae of both 
eyes similtaneously (fig. 6). It is very 
probable that these birds rely entirely 
on the degree and rate of convergence 
of the eyes in achieving the fine judg- 
ment of the distance and relative speed 
of their prey which they obviously 
possess. To achieve the necessary 
overlap of the visual fields, their eyes 
Man is fully binocular, his eye move- 
ments are conjugate and extensive 
and his visual field (at daylight inten- 
sities) is being continually scanned as 
these movements cause successive small 
areas of the field to be fixated foveally 
by both eyes. Though vision appears 
to be continuous, sensation is, in fact, 
almost completely suppressed during 
these scanning movements, and is only 
experienced in the intervals when the 
eye is stationary. (It is impossible to 
see one’s Own eyes moving in a mirror.) 
This suppression is probably neces- 
sary to prevent confusion between 
movement of the image across the 
fovea due to eye movement and move- 
ment of the image due to movement 
