202 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 194. 



Proceeding inward from this external por- 

 tion of the ear, is a tube, something more than 

 an inch long, terminating in an oval-shaped 

 opening, h, across which is stretched an elas- 

 tic membrane, like the parchment on the head 

 of a drum. This oval-shaped opening has re- 

 ceived the name of the tympanum, or drum of 

 the ear, and the membrane stretched across it 

 is called the "membrane of the tympanum, or 

 drum of the ear." 



The sound concentrated at the bottom of the ear-tube falls upon the mem- 

 brane of the drum, and causes it to vibrate. That its motion may be free, 

 the air contained within and behind the drum has free communication with 

 the external air by an open passage, / called the eustachian tube, leading to 

 the back of the mouth. A degree of deafness ensues when this tube is ob- 

 structed, as in a cold; and a crack, or sudden noise, with immediate return 

 of natural hearing, is generally experienced when, in the eflbrt of sneezing or 

 otherwise, the obstruction is removed. 



The vibrations of the membrane of the drum are conveyed further inward, 

 through the cavity of the drum, by a chain of four bones (not represented in 

 the figure on account of their minuteness), reaching from the center of tho 

 membrane to the commencement of an inner compartment which contains tho 

 nerves of hearing. This compartment, from its curious and most intricate 

 Btructure is called the Labyrinth. Fig. 194, c e d. 



The Labyrinth is the true ear, all the 

 other portions being merely accessories by 

 which the sonorous undulations are propa- 

 gated to the nerves of hearing contained 

 in the labyrinth, which is excavated in the 

 hardest mass of bone found in the whole 

 body. Fig. 195 represents the labyrinth 

 on an enlarged scale, and partially open. 



The labyrinth is filled with a liquid sub- 

 stance, through which the nerves of hearing 

 are distributed. "When the membrane of 

 the drum of the ear is made to vibrate by the undulations of sound striking 

 against it, the vibrations are communicated to the little chain of bones, 

 which, in turn, striking against a membrane which covers the external 

 opening of the labyrinth, compresses tho liquid contained in it. This ac- 

 tion, by the law of fluid pressure, is communicated to the whole interior of 

 the labyrinth, and consequently to all portions of the auditory nerve dis- 

 tributed throughout it: the nerve thus acted upon conveys an impression to 

 the brain. 



The several parts of the labyrinth consist of what is called the vestibule, 

 e. Fig. 194, three semicircular canals, c, imbedded in the hard bone, and a 

 winding cavity, called the cochlea, d, like that of a snaU-shell, in which fibres, 



Fig. 195. 



