The science of life : fully illustrated in tone and line and including many diagrams

BOOK 1

upper faces, and they are surrounded by the endings of nerve-fibres ; moreover, they are placed on an elastic membrane and overhung by a rigid shelf. The working of the ear depends entirely on these anatomical relationships. When the fluid surrounding the apparatus vibrates, the elastic membrane

Fig. 45. The spiral cochlea cut across.

in bone. (Compare Fig. 52.) bounces up and down ; this brings the sensecells into collision with the rigid shelf, and it is their impact with the shelf that stimulates the cells and. causes them to dispatch impulses along the nerve-fibres to the brain.

According to the theory most extensively held (for there are differences of opinion here) the method of S discriminating between sounds of different pitch is as follows. The diameter of the cochlea decreases steadily from end to end of the spiral ; it is a spiral staircase that dwindles to a point at the top. Since all the structures inside it decrease in proportion, the width of the elastic membrane on which the sense-cells are placed also decreases. That is to say, since the period of vibration of an elastic body varies with its size, the different parts of this membrane are tuned to different notes. ‘Ihe apparatus is like a piano or a harp, where the wires give higher notes as they decrease in length. Now it is well known

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Neros fibres

Fig. 46.

THE SCIENCE OF LIFE

It is embedded

CHAPTER 3

that if one sounds a tuning-fork near a piano with the damping pedal down, the particular note of the tuning-fork is echoed by the appropriate wire of the piano—the pulsations emitted by the tuning fork make a wire vibrate if it is tuned to the same pitch. And in a similar way rhythmical vibrations of the fluid in the inner ear only shake the particular part of the elastic membrane that is tuned to their own pitch. Notes of different pitch affect different parts of the membrane, and _ so cause impulses to be sent to the brain along different nerve-fibres. Clearly, there is no distinction at all in the manner in which C and C sharp stimulate cells—both notes cause sensitive cells to be bumped against an overhanging shelf, and the distinction lies in the fact that each note shakes a different group of cells.

Thus we see that the auditory cells are not sensitive to sounds as such, but they are sensitive to being touched ; the ear is an ingenious device for touching different cells when notes of different pitch are sounded. The nerve-fibres run from the ear to the brain, and here the impulses are interpreted as notes of varying pitch according to the particular fibres along which they arrive. Further, it is in the brain that the sensations given by simultaneous notes of different pitch are combined to form the complex sensation of a chord, and becoming pleasant, discordant,

Elastic’' Membrane

A few sensitive cells of the ear, to show how they are situated.

rousing, and so forth, acquire emotional significance.

Sight. In studying hearing and smell we were forced to make humiliating confessions of inferiority. Dogs perceive sounds and scents too faint for us to notice, and the human nose and outer ear are miserably developed structures when compared with