320 
The extent to which an owl’s eye is 
better than man’s at night is roughly 
calculable. It is unlikely that the 
sensitivity of the owl’s eye is more than 
100 times that of the fully dark-adapted 
human eye, so that an owl perhaps 
sees nearly as well by bright starlight 
as man by a bright full moon. But 
though bright moonlight may look 
light enough to play tennis by, it is not 
so in fact, and it is probable that an 
owl’s chances of finding a mouse on a 
dark night by sight alone are nil. If 
his eyes enable him only to avoid 
collision with bushes and other ob- 
stacles, they serve him very well.? 
The Organs of Hearing 
Far less is known of the comparative 
structure of the ears of birds than of 
the eyes. They are hidden, secret 
things, lacking the large external pinna 
of most mammals and, except in os- 
triches and a few other very bald spe- 
cies, quite invisible externally. Only a 
close scrutiny discloses that the feathers 
behind the angle of the jaw have a 
slightly different sheen owing to the 
absence of barbules; and these feathers 
must be turned forward before the 
small external aperture of the outer 
ear can be seen. 
The avian ear is divisible, like the 
human, into outer, middle and inner 
parts (fig. 9). The cavity_of the 
outer ear, the external meatus, is 
terminated by the tympanic mem- 
brane. The middle ear cavity commu- 
nicates with the pharynx through the 
Eustachian tube and with the inner ear 
by two membrane-covered openings, 
9 The view of Vanderplank (1934) that 
owls “‘see”’ their prey as self-luminous sources 
of infrared rays has received considerable 
publicity. It may be noted that Hecht and 
Pirenne (1940) found that the pupillomotor 
response of the eye of Asio wilsonianus to dif- 
ent wave lengths yields a curve identical with 
the human scotopic brightness curve. Mat- 
thews and Matthews (1939) have shown that 
the eye of Strix aluco (Vanderplank’s species) 
does not transmit infrared, and that no elec- 
trical response follows exposure to infrared. 
It may be concluded that Vanderplank’s 
theory has no foundation in fact. 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
the oval and round windows. The 
oval window is almost completely 
filled by the foot plate of the columella, 
a bone which, with its cartilaginous 
extension, the extra-columella, com- 
municates the movements of the 
tympanic membrane to the liquid 
perilymph of the inner ear. 
The auditory part of the inner ear, 
the cochlea, resembles that of man in 
its general plan, though it is propor- 
tionately much shorter and is straight, 
or nearly so, instead of being rolled 
into a tight spiral. It is divided into 
two channels by cartilaginous shelves 
supporting the basilar membrane, ex- 
cept near the apex, where the channels 
are connected by a narrow opening, 
the helicotrema; and these two chan- 
nels communicate with the round 
and oval windows respectively. Any 
displacement of the perilymph at the 
oval windows is compensated by a 
displacement in the opposite sense 
at the round window, and this neces- 
sarily involves a displacement of the 
basilar membrane unless the move- 
ment is so slow that the pressure in the 
channels can be continually equalized 
by flow through the _helicotrema. 
On the basilar membrane are ranged 
the hair cells, which are believed to 
be the actual receptors for sound. 
They carry fine processes which termi- 
nate on or in the tectorial membrane 
and are consequently distorted or 
stretched when the basilar membrane 
is displaced. The hair cells are 
innervated by nerve fibers from the 
cochlear ganglion. The whole of this 
sensory apparatus is separated from 
the perilymph by the basilar mem- 
brane below and the tegmentum vas- 
culosum !° above. 
The basilar membrane contains 
transverse fibers which increase regu- 
larly in length and probably decrease 
in tension from the outer (basal) end of 
the membrane to the apical end. It is 
believed that, as in man, these fibers 
10 The tegmentum vasculosum _corre- 
sponds roughly in position to Reissner’s 
membrane in the mammalian cochlea but 
incorporates the stria vascularis. 
