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CHAPTER XVIII, 



OF HEARING. THE ORGAN OF HEARING. ITS DEVELOPMENT IN THE 



ANIMAL SERIES. THE EXTERNAL EAR. THE TYMPANUM. THE 



LABYRINTH. THE FUNCTIONS OF THESE PARTS. 



IT is by the sense of hearing that the mind takes cognizance of 

 those oscillations of elastic matter which give rise to the phenomena 

 of sound. 



The communication of these oscillations to the ear may take 

 place through the air, or through the intervention of some solid 

 conductor, brought into immediate connexion with the organ of 

 hearing. 



The essential part of the organ of hearing is a sac, containing 

 fluid, upon which the nerve of hearing is freely distributed : this 

 sac being in connexion with the cranial parietes. This is repre- 

 sented in the human subject by that small cavity which is excavated 

 in the petrous portion of the temporal bone, called the vestibule. 

 This, and three semi-circular canals, with a spirally disposed canal, 

 divided by a partition, constituting the cochlea, form the labyrinth. 

 External to this, and situate between the squamous and petrous 

 portions of the temporal bones is a cavity, the tympanum, which in 

 front further communicates very freely with the cavity of the throat 

 through an open channel, the Eustachian tube, whereby air has a free 

 access into the tympanum. This cavity is closed on the outside by 

 a membrane (membrana tympani) which extends over its external 

 orifice as over a drum. A communication is established between 

 the membrane and the inner wall of the tympanum, by a chain of 

 small bones which extends from the one to the other. These are 

 the ossicles of the ear. The outer bone of the chain is intimately 

 attached to the membrana tympani, and the inner one to a mem- 

 brane which closes the vestibule on the outside. The three bones 

 which compose the chain are articulated by moveable joints, and 

 are moved by small muscles, which are thus enabled to regulate the 

 tension of the membrana tympani, as well as of the membrane of 

 the vestibule. Externally is an apparatus for collecting sounds and 

 conducting them to the tympanum. 



Development of the Organ of Hearing in the Animal Series There is no 

 organ in the body in which we find a more remarkable gradation of develop- 



