142 PHYSIOLOGY AND TEMPERANCE. 



vestibule at each end. The cochlea, which has been compared 

 to a winding-stair or a snail shell, is in front of the vesti- 

 bule. In the cochlea are minute fibres, or tendrils, regularly 

 arranged from bottom to top, which may be likened to the 

 key-board of a piano. The cochlea is filled with fluid, and 

 in it floats a delicate sac containing hair-like bristles, some 

 fine sand, and two ear-stones. In this wonderfully con- 

 structed chamber are the endings of the nerve of hearing, 

 spread out on the walls of the cochlea. 



21. How we Hear. When one throws a stone in the 

 water, from the point where the stone strikes are seen a 

 series of circular wavelets. In like manner, when two sub- 

 stances strike together, waves of air are produced. These 

 waves, caught by the outer ear, pass through the auditory 

 canal and strike upon the drum. The impulse is conveyed 

 through the chain of bones into the vestibule, and on to the 

 cochlea. This sets the bristles, stones and sand in motion 

 in the liquid, more or less, according to the intensity of 

 the air wave. The effect of the impulse upon the nerve by 

 agitation of the contents of the cochlea is conveyed to the 

 brain, and we hear. 



A knowledge of the principles of sound would be necessary 

 to understand how the various noises and musical sounds are 

 created. The loudness of a sound depends upon the force of 

 the air waves. Other variations depend upon the length, 

 frequency and regularity of the waves. 



Although sound is mostly carried by the air waves, as 

 described, it may reach the cochlea through the bones of the 

 head. A watch can be distinctly heard when placed on the 

 top of the head, or if we hold it between the teeth the sound 

 is carried to the nerves of hearing. Deaf persons by this 

 means may be made to hear, unless the nerve itself is de- 

 stroyed. The ability to recognize distance in sound, and the 

 direction from which it comes, is largely due to experience 

 and to education of the sense of hearing. 



