Fibrils of the Ear and Eye.
Pianos, organs, and other musical instruments yield their notes by the vibration of strings, pipes, or reeds of definite size and form. Across the larynx, the box-like organ of the throat, the vocal cords vibrate in an identical way. When we sing a note into an open piano, the string capable of giving out that note at once responds. Helmholtz believed that in the ear the delicate, graduated structures, known as the rods of Corti, vibrate in the same way when sound-waves reach them, giving rise to auditory impressions. Analogous in operation are the fibrils of the eye which respond to light-waves of various length and intensities. The human eye has muscles which modify its globularity, rendering its lenses more or less convex. A cat has a higher degree of this kind of ability, so that it can dilate its pupil so much as to see clearly in a feeble light. A man who remains in a darkened room so rests his nerves of vision that in four or five hours he can readily discern what would be unseen were he newly brought into the darkness.