THE INTERNAL EAR. 823 



Fig. 261 (1, 3, 4). This is a spiral canal, about an inch and a half long, and one-tenth of 

 an inch wide at its commencement, gradually tapering to the apex, and making, in its 

 course, two and a half turns. Its interior presents a central pillar, around which winds 

 a spiral lamina of bone. The fenestra rotunda (2, Fig. 261), closed in the natural state 

 by a membrane (the secondary membrana tympani), lies between the lower portion of the 

 cochlea and the cavity of the tympanum. 



FIG. 261. T/ie left bony labyrinth of a new-born child, forward and outward view. From a photograph. 



(Rudinger.) 



1, thfcwide canal, the beginning of the spiral canal of the cochlea; 2, the feneotra rotunda: 8. the second turn of the 

 cochlea; 4, the final half-turn of the cochlea; 5, the border of the bony wall of the vestibule, situated l.ctwi-cn the 

 cochlea and the semicircular canals; 6, the superior, or sagi tal semicircular canal; 7, the portion of the superior 

 semicircular canal bent outward; 8, the posterior, or transverse semicircular caual; 9, the portion of the ptrri<>r 

 connected with the superior semicircular canal; 10. point of junction of the superior and the posterior semicir'-u- 

 lar canal; 11, the ampulla ossea externa; 12, the horizontal, or external semicircular canal. The explanation of 

 this Figure has been modified and condensed from Rudinger. 



What is called the membranous labyrinth is contained within the bony parts just 

 described. Its structure, and the ultimate distribution and connections of the auditory 

 nerve, which penetrates by the internal auditory meatus, involve some of the most intri- 

 cate and difficult points in the whole range of minute anatomy. Some of these ha\v 

 direct and important relations to the physiology of hearing, while many are of puivly 

 anatomical interest. Such facts as bear directly upon physiology will be considered fully 

 in connection with the functions of the internal ear. 



Physics of Sound. 



The sketch that we have given of the general anatomical arrangement of the auditory 

 apparatus conveys an idea of the uses of the different parts of the ear. The waves of 

 sound must be transmitted to the terminal extremities of the auditory IHTVO in the 

 labyrinth. These waves are collected by the pinna, are conducted to the moml.rana 

 tympani through the external auditory meatus, produce vibrations of the ID. ml.r.-uia 

 tympani, are conducted by the chain of ossicles to the opening in the labyrinth, and 

 are communicated through the fluids of the labyrinth to the ultimate nervous lilanu-nN. 

 The free passage of air through the external meatus and the communication* of the cavity 

 of the tympanum with the mastoid cells, and, by the Eustachian tube, with the pharynx, 

 are necessary to the proper vibration of the membrana tympani; the iiite.irrity of tho 



