ok off their hats when it
commenced, are physically deficient. Their Corti rods cannot be properly
developed.

What applies to one single note applies also to the elements of a
musical chord. A dozen notes may sound simultaneously, but the ear is
able to assimilate each and blend it with its fellows; yet it requires a
very sensitive and well-trained ear to pick out any one part of a
harmony and concentrate the brain's attention on that part.

The ear has a much larger range than the eye. "While the former ranges
over eleven octaves, but little more than a single octave is possible to
the latter. The quickest vibrations which strike the eye, as light, have
only about twice the rapidity of the slowest; whereas the quickest
vibrations which strike the ear, as a musical sound, have more than two
thousand times the rapidity of the slowest."[26] To come to actual
figures, the ordinary ear is sensitive to vibrations ranging from 16 to
38,000 per second. The bottom and top notes of a piano make respectively
about 40 and 4,000 vibrations a second. Of course, some ears, like some
eyes, cannot comprehend the whole scale. The squeak of bats and the
chirrup of crickets are inaudible to some people; and dogs are able to
hear sounds far too shrill to affect the human auditory apparatus.

Not the least interesting part of this wonderful organ is the tympanic
membrane, which is provided with muscles for altering its tension
automatically. If we are "straining our ears" to catch a shrill sound,
we tighten the membrane; while if we are "getting ready" for a deep,
loud report like that of a gun, we allow the drum to slacken.

The _Eustachian tube_ (Fig. 134) communicates with the mouth. Its
function is probably to keep the air-pressure equal on both sides of the
drum. When one catches cold the tube is apt to become blocked by mucus,
causing unequal pressure and consequent partial deafness.

Before leaving this subject, it will be well to remind our more youthful
readers that the ear is delicately as well as wonderfully made, and must
be treated with respect. Sudden shouting into the ear, or a playful
blow, may have most serious effects, by bursting the tympanum or
injuring the arrangement of the tiny bones putting it in communication
with the inner ear.

MUSICAL INSTRUMENTS.

These are contrivances for producing sonorous shocks following each
other rapidly at regular intervals. Musical sounds are distinguished
from mere noises by