HEARING. 605 
It will be assumed that the student has made himself famil- 
iar with the general anatomy of the ear. The essential points 
in regard to sound are considered in the chapter on “The 
Voice.” It will be remembered that what we term a musical 
tone, as distinguished from a noise, is characterized by the 
regularity of vibrations of the air that reach the ear; and that 
just as ethereal vibrations of a certain wave-length give rise to 
the sensation of a particular color, so do aérial vibrations of a 
definite wave-length originate a certain tone. In each case must 
we take into account. a physical cause for the physiological 
effect, and these bear a very exact relationship to one another. 
As will be seen later, while in all animals that have a well- 
defined sense of hearing the process is essentially such as we 
have indicated above, the means leading up to the final stimu- 
lation of the end-organ are very various. At present we shall 
consider the acoustic mechanism in mammals, with special ref- 
erence to man. There are in fact three sets of apparatus: (1) 
Fic. 445.—Section through auditory organ (after Sappey). 1, pinna; 2, 4,5, cavity of concha, 
external and auditory meatus with opening of ceruminous glands; 6, membrana Hemepent 
7, anterior part of incus ; 8, malleus; 9, long handle of malleus, attached to internal sur- 
face of tympanic membrane—it is here represented as strongly indrawn ; 10, tensor tym- 
pani muscle; 11, tympanic cavity ; 12, Eustachian tube ; 13, superior semicircular canal ; 
4, posterior semicircular canal ; 15, external semicircular canal’; 16, cochlea ; 17, internal 
auditory meatus; 18, facial nerve; 19, large petrosal nerve; 20, vestibular branch of 
auditory nerve ; 21, cochlear branch of same. 
one for collecting the aérial vibrations; (2) one for transmit- 
ting them; and (3) one for receiving the impression through a 
fluid medium; in other words, an external, middle, and internal 
ear. 
