Hearing, and the Ear. 455 



the mollusk by means of the ingoing current of water, and their con- 

 tinued growth to maturity there includes the entire atrophy of their eyes 

 so that not even a pigment spot is left. There, however, the} 7 reproduce 

 their larvae with complete eyes, and turn them off into the water to look 

 out for themselves, which they do by repeating the ancestral example. 

 ( Semper). This case is like that of the mole ; and if either animal 

 were forced to finish their whole lives in light, after some generations 

 they would cease to lose their eyes. 



CHAPTER XL VIII. 



HEARING, AND THE EAR. 



The sense of hearing depends upon the fact, that vibrations of a cer- 

 tain sort are able when they strike the ear, to set up in it a motion which 

 it is able to communicate to the auditory nerve in the shape of a nerve 

 current, which flowing to the brain excites in certain brain cells the 

 motion we call hearing. Whatever the nature of the motion, after it ag- 

 itates the ear, the nerve and the cell, it is certain that before it reaches 

 the ear, it includes vibrations of some ponderable substance, commonly 

 the air. As stated in chapter 39, the pitch of these vibrations runs 



FIG. 191. Ear, natural size. 



A Outside ear or conch (or flap ), D Utriculus with Canals. 



B. Ear bones. #. Cochlea. 



C\ Tympanum drum-head. f . Eustachian tube opening into phar- 



ynx. (After Helmholtz.) 



from about 33 up to 30,000 or 40,000 per second. The essential part 

 of the ear, in most animals, is a sac of water which receives the vibra- 

 tions, and connected with which is the auditory nerve, which conveys 

 the stimulus to the brain. In very few is the organ confined to this 

 simplicity of construction. Various appendages 'are found in the ears 

 of the more highly developed animals, which serve to intensify the sen- 

 sation, to make it more acute, and finally to make it discriminating, 

 that is to have a different sensation for different qualities and pitch of 

 sound. A description of the human ear will serve as a good basis for 

 study and standard of comparison for others. 



