SENSE ORGANS OF 
ulation about its performance and the 
part it plays in determining avian be- 
havior. This is far less true of the 
ear, which offers a most promising and 
unjustly neglected field for experiment 
and observation. 
Other Sense Organs 
Of the other sense organs, there 
remains little that is worth saying. 
The sense of smell is undoubtedly 
only very moderately developed by 
comparison with the more gifted 
mammals. ‘Though birds can be 
trained to respond to odoriferous sub- 
stances, such training is very readily 
disrupted by visual or aural stimuli, 
which is at least an indication of the 
relative unimportance of olfaction. 
It should be noted that there are per- 
sistent reports that some birds show 
evidence of a keener sense of smell 
than might be expected. These in- 
clude the carrion feeders, especially 
the vultures, scavengers like gulls 
and petrels and, rather surprisingly, 
the ducks. The literature, which is 
largely anecodotal, has been reviewed 
by Gurney (1922) in this journal. 
Most of the positive evidence brought 
forward by one authority has been 
destructively criticized by another. 
Indeed, in this field critical experi- 
ments are almost impossible, because 
human beings labor under the ex- 
cessive difficulty of not knowing what 
to look for. There is no theory of 
smell which is even moderately con- 
sistent with the facts of human ol- 
factory experience, and we can per- 
haps best sum up the situation for 
birds by saying we know even less 
about their sense of smell than we do 
about our own. 
The sense of touch is in a class rather 
by itself. For one thing it is not a 
single sense in the way in which sight 
and hearing are (in vertebrates) 
single senses; there is no organ of 
touch, but a multiplicity of organs 
whose individual responses to stimu- 
lation must be centrally integrated 
before a man can tell a tennis ball 
from an egg with his eyes shut. For 
BIRDS—PUMPHREY 325 
another, though not distance recep- 
tors, the organs of touch are comple- 
mentary to them. Man and the 
primates learn very much by seeing 
plus handling and by hearing plus 
handling, and it is in the skin of the 
fingers and the muscles and joints of 
the arms that the sense of touch is 
most highly developed. We find in 
the inner skin of the hands numerous 
sensory end-organs of a type which 
are rare or absent elsewhere, con- 
sisting of a capsule of tissue within 
which the fiber of a sensory nerve 
terminates in a coil or web. In other 
mammals which have no hands but, 
like the pig and the mole, use the 
snout for exploratory purposes, we 
find very similar encapsulated end- 
organs most numerous in the moist 
mucosa round the nostrils. In birds 
similar end-organs are found in the 
beak and tongue, and though at first 
sight the horny beak appears unlikely 
to be a suitable vehicle for a refined 
sense of touch, the presence of end- 
organs of this type suggests that it is 
in fact the part of birds which is tact- 
ually the most sensitive. The sensory 
capsules are most complex and most 
numerous, so far as known, in the bills 
of ducks (pl. 4) and geese, and very 
likely are important for finding food 
in mud. It does not seem that the 
histology of the beaks of waders has 
ever been examined. By contrast the 
innervation of the feet is scanty (even 
in the hawks and owls), and they are 
probably quite insensitive to touch. 
Incertae Sedis 
This article is about sense organs, 
and it might be the better part of 
wisdom for the author to leave it at 
that and refrain from even passing 
mention of senses which in some way 
have a reality and importance for 
animals in general and birds in par- 
ticular but which are not, or at least 
not yet, referable with certainty to 
special sense organs. These “senses” 
are the sense of time and the sense of 
direction; and their relevance to the 
migratory problems of birds is obvious, 
