AUDITORY KERVES. 815 



the increased flow of this secretion from emotional causes, which probably operate through 

 the sympathetic, a hypersecretion almost immediately follows irritation of the mucous 

 membrane of the conjunctiva or of the nose. The same result follows violent muscular 

 effort, laughing, coughing, sneezing, etc. The secretion of tears under stimulation of the 

 mucous membrane is reflex. 



CHAPTER XXY. 



A UDITION. 



Physiological anatomy of the auditory nerves General properties of the auditory nerves Topographical anatomy 

 of the parts essential to the appreciation of sound The external ear General arrangement of the parts composing 

 the middle ear Anatomy of the tympanum Arrangement of the ossicles of the ear Muscles of the middle ear 

 Mastoid cells Eustachian tube Muscles of the Eustachian tube Mucous membrane of the middle ear and of 

 the Eustachian tube General arrangement of the bony labyrinth Laws of sonorous vibrations Noise and musi- 

 cal sounds Intensity, pitch, and quality of musical sounds Musical scale Harmonics, or overtones Eesonators 

 of Helmholtz Kesultant tones Summation tones Harmony Discord Tones by influence (consonance) Uses 

 of different parts of the auditory apparatus Uses of the external ear Structure of the membrana tympani Uses 

 of the membrana tympani Vibrations of the membrane by influence Appreciation of the pitch of tones Mech- 

 anism of the ossicles of the ear Physiological anatomy of the internal ear General arrangement of the mem- 

 branous labyrinth Vestibule Semicircular canals Cochlea Liquids of the labyrinth Distribution of nerves in 

 the cochlea Organ of Corti Functions of different parts of the internal ear Functions of the semicircular canals 

 Functions of the parts contained in the cochlea Summary of the mechanism of audition. 



THE general considerations introductory to the study of vision are equally applicable 

 to the physiology of hearing. The impressions of sound are conveyed to the brain by 

 special nerves ; but, in order that these impressions shall reach these nerves so as to be 

 properly appreciated, a complex accessory apparatus is required, the integrity of which is 

 essential to perfect audition. The study of the arrangement and action of these accessory 

 parts is even more important and is far more intricate than the physiology of the auditory 

 nerves. The latter simply convey the impressions to the brain, by a mechanism analogous 

 to that of general nervous conduction, the essential character of which is not fully under- 

 stood. The auditory nerves conduct impressions of sound, as the optic nerves conduct 

 impressions of light ; and this statement expresses the extent of our positive knowledge ; 

 but there is an elaborate apparatus by which the waves are collected, conveyed to a 

 membrane capable of vibration, and finally carried to the nerves, by which we are enabled 

 to appreciate the intensity and the varied qualities of sound. 



Our positive and definite knowledge of the structure and arrangement of the 'auditory 

 apparatus is by no means so complete as it is with regard to the eye, nor do we as yet 

 understand so clearly the physiological relations of many points developed by late ana- 

 tomical researches ; and, for this reason, it does not seem desirable to consider the struct- 

 ure of the ear as fully as we have the anatomy of the eye, restricting ourselves, as we 

 have done, to the physiological anatomy of parts. With this end in view, we shall take 

 up fully the following points : 



1. The physiological anatomy and the general properties of the auditory nerves. 



2. The physiological anatomy of the parts essential to the correct appreciation of 

 sound. 



3. The laws of the propagation of sonorous vibrations, as far as they are applicable 

 to audition. 



4. The physiological action of different parts of the auditory apparatus. 



Physiological Anatomy of the Auditory Nerves. The auditory nerve constitutes the 

 portio mollis of the seventh pair of Willis. The origin of this nerve can easily be traced 

 to the floor of the fourth ventricle, where it presents two roots. The external, or super- 



