616 ANIMAL PHYSIOLOGY. 
association. Sounds are, however, always referred to the world 
without us. The animals with movable ears greatly excel man 
in estimating the direction, if not the distance, of sounds, 
There are few physiological experiments more amusing than 
those performed on a person blindfolded, when attempting to 
determine either the distance or the direction of a sounding 
tuning-fork, so gross are the errors made. 
One who makes such observations on others may notice 
that most persons move the ears slightly when attempting to 
make the necessary discriminations, which of itself tends to 
show how valuable mobility of these organs must be to those 
animals that have it highly developed. 
Range of Auditory Discrimination.—If we compare the range 
of sense-perception of eye and ear, we find that the latter is in 
this respect far superior to the former. Assuming that the 
perception of red is owing to the influence of rays of light with 
four hundred and fifty-six billions of vibrations per second and 
violet at the opposite end of the spectrum with rays of six hun- 
dred and sixty-seven billions, it will be seen that the total range 
does not correspond with even one octave; while the ear can 
discriminate between tones answering on the one hand to about 
forty aérial vibrations per second and on the other to thirty- 
eight thousand or more, though this latter is greater in the 
upward direction than most persons can appreciate. Such lim- 
its answer to at least ten times that for the eye. On the other - 
hand, a sense-impression on the organ of hearing lasts a shorter 
time by far than in the case of the eye, so that fusion of audi- 
tory sense-impressions is less readily produced. 
SPECIAL CONSIDERATIONS. 
Comparative——A mong invertebrates steps of progressive de- 
velopment can be traced. Thus, in certain of the jelly-fishes we 
find an auditory vesicle (Fig. 459) inclosing fiuid provided with 
one or more otoliths or calcareous nodules and auditory cells 
with attached cilia, the whole making up an end-organ connected 
with the auditory nerve. A not very dissimilar arrangement 
of parts exists in certain mollusks (Fig. 460). The vesicle may 
lie on a ganglion of the central nervous system. On the other 
hand, the vesicle may lie open to the exterior, as in decapod 
crustaceans; and the otoliths be replaced by grains of sand 
_from without. It is difficult to decide what the function of 
otoliths may be in mammals; but there seems to be little reason 
