CHAP, xvin.] HEARING. 87 



muscle receives a branch from the facial nerve. The anastomosis of 

 Jacobson results from the subdivision of the tympanic branch of the 

 glosso-pharyngeal nerve, which enters the cavity of the tympanum 

 below, and passing over the promontory gives off branches to the 

 membranes of the fenestrae, and Eustachian tube, and to the otic 

 ganglion. 



A branch is described by Arnold as proceeding from the otic 

 ganglion to the tensor tympani muscle. 



The external ear is supplied by the facial nerve as regards its mus- 

 cular apparatus, and by the fifth pair as regards its sentient surfaces. 



The influence of the facial nerve upon the muscular apparatus of 

 the organ of hearing, whether tympanic or labyrinthic, is similar to 

 that of the third nerve upon the muscles of the eyeball, or upon the 

 iris and ciliary muscle. And it seems probable that while volition 

 can exercise a certain influence upon the muscular apparatus of 

 hearing, that apparatus may likewise be excited to action through 

 the physical stimulus of sound affecting the auditory nerve, which 

 re-acting upon the portio dura excites its fibres to a degree pro- 

 portionate to the intensity of the sound; as the stimulus of light 

 affecting the optic nerve re-acts upon the iris. 



We shall now proceed to inquire into the office of each part of the 

 complex organ of hearing-."* 



* The following points respecting the laws of sound should be borne in mind 

 in considering the offices of the various parts of that complex acoustic appa- 

 ratus, the human ear. 



1. Any irregular impulse communicated to the air will produce a noise: a 

 succession of impulses occurring at exactly equal intervals of time, and exactly 

 similar in duration and intensity, constitutes a musical sound. 



2. The frequency of repetition necessary for the production of a continued 

 sound from single impulses is, probably, generally not less than sixteen times 

 in a second, but Savart thinks that some ears may distinguish a sound result- 

 ing from only ten or eight vibrations in a second. Or the other hand, sounds 

 are audible which consist of 24,000 vibrations in a second. 



3. Sound may be propagated or conducted by air, gases, liquids and solids, 

 with various degrees of rapidity. 



4. Sound travels through air at the temperature of 62 Fahr. at the rate of 

 1125 feet in a second. 



5. Sound is incapable of transmission through a vacuum. 



6. The propagation of sound is the more effectively performed as the me- 

 dium of transmission is more dense. Rarefied air, gases of low density, and 

 soft solids, are less perfect conductors of sound than much denser materials of 

 the same kind. 



7. We distinguish in sounds, 1, the pitch; 2, the intensity or loudness ; 

 3, the quality or timbre. 



The pitch of the sound depends on the rapidity with which the vibrations 



