SENSE ORGANS OF 
turnal mammals. The increase in 
sensitivity is achieved at the sacrifice 
of many of the features which increase 
the resolving power of the diurnal 
eye. In the retina rods are predomi- 
nant and cones few, the oil droplets 
are colorless or absent, and there is 
no fovea or only a vestige. The curva- 
ture of the cornea is less and its area 
greater, and the lens approaches a 
spherical shape and retreats toward 
the fundus of the eye (fig. 8). The 
apparatus of accommodation is de- 
generate. The effect of all these 
changes in dimensions is to produce a 
BIRDS—-PUMPHREY 
319 
the brilliant ‘‘eyeshine”’ of cats caught 
in a car’s headlights. Owls rather sur- 
prisingly have no tapetum,’ but the 
nightjar family, which show brilliant 
eyeshine, presumably have. Apart 
from these two families, nocturnal 
habits are rare and sporadic in birds. 
The kiwi is exceptional in this, as in 
so many other ways, but its eyes are 
considered to be very poor. Alone 
among birds it is believed to have a 
keen sense of smell and to find its 
food thereby. 
In spite of all that has been said 
about the night vision of owls, it does 
Ficure 8.—Horizontal sections of the left eyes of the diurnal honey buzzard Pernis apivorus 
and the nocturnal Australian frogmouth Podargus strigoides. 
substantially smaller, and therefore, 
brighter (though more aberrated and 
distorted) image. 
A typically nocturnal adaptation 
in mammals is the formation of a 
tapetum. In diurnal eyes the retina 
is backed by the densely pigmented 
chorioid layer of cells whose function 
is to absorb light not absorbed by the 
retina, and so prevent blurring of the 
image by scattering, the effect known 
to photographers as halation. Noc- 
turnal eyes can usually not afford 
such a loss of radiant energy in the 
pigment which is replaced by a mirror 
called the tapetum over at least part 
of the fundus of the eye, so that light 
which is passed unabsorbed through 
the retina in the first instance is 
reflected back into it. It is the pres- 
ence of a tapetum which gives rise to 
(After Franz, 1934.) 
not seem possible that the most noc- 
turnal of them hunt by sight alone. 
8 The only use of a tapetum is to give the 
retina a second chance, and it is only when 
the retina itself is relatively inefficient that a 
tapetum is worth while. If, for example, 
the rods transmit 1 percent and absorb 99 
percent of the light falling on them, a tape- 
tum which would allow them to absorb a 
further 99 percent of the 1 percent which 
escaped them at the first transmission would 
not be economic since it would increase the 
efficiency by only 1 percent. If, however, 
the rods only absorb 50 percent at the first 
transmission and a further 50 percent of the 
light reflected back into them by a tapetum, 
then if the tapetum is a perfect reflector, it 
will increase the efficiency by 50 percent, 
which is worth while. 
The terminal segments of the rods in the 
owl’s retina are known to be exceptionally 
long and, therefore, may possibly absorb 
so much incident light that a tapetum 
would be a luxury. 
