THE SENSE OF HEARING. 



371 



loudly when the ears are closed than when open. The vibrations thus con- 

 ducted probably partly affect the internal ear directly, and partly indirectly bv 

 setting in oscillation the tympanic membrane. It is said that when the sound 

 of a tuning-fork held close to the ear dies away, it may again be heard if the 

 handle of the fork be pressed against the teeth. When the tone now fails, 

 it once more becomes audible if one of the ear-passages is lightly closed, and 

 the sound seems to be on the side which is closed. The sensation failing, it 

 may again be aroused if the appropriately formed handle of the fork be 

 inserted in the auditory meatus. 1 



Normal individuals differ greatly in their keenness of hearing, and tests 

 show frequently disparity in the sensibility of the two ears. The hearing 

 ability of children is said to improve up to the age of twelve years. There 

 is no functional relation between keen hearing and sensibility to pitch. 2 



The Internal Ear, or Labyrinth. — The internal ear is the site of the true 

 organ of hearing. The membranous labyrinth (PI. 1, Fig. 4 ; Fig. 191) is a com- 

 plicated system of membranous tubes and sacs, in which terminate at particular 

 points the filaments of the auditory nerve ; it is contained within a chamber, 

 the bony labyrinth, hollowed out in the petrous bone. The cavity of the bony 

 labyrinth (Figs. 188, 189) consists of a median part, the vestibule, which is pro- 

 longed posteriorly in the system of semicircular canals and anteriorly in the 

 cochlea. The vestibule is a space which measures about oue-fifth of an inch 

 in diameter, and it is perforated in its outer wall by an oval opening known 

 as the fenestra ovalis. The semicircular canals are three tubes of circular 



Fig. 188.— Right bony labyrinth, viewed from 

 outer side: the figure represents the appearance 

 produced by removing the petrous bone down to 

 the denser layer Immediately surrounding the 

 labyrinth (from Quain, after Sdmmering): 1, 2,3, 

 the superior, posterior, and horizontal semicir- 

 cular canals; 4, 5, 6, the ampullae of the same; 

 7, the vestibule; 8, the fenestra ovalis; 9, fenestra 

 rotunda ; 10, first turn of the cochlea ; 11, second 

 turn ; 12, apex. 



Fig. 189.— Interior view of left bony labyrinth after 

 removal of the superior and external trails (from 

 Quain, after Sdmmering): I, '-', ::, the superior, pos- 

 terior, and horizontal semicircular canals; i, fovea 

 hemi-elliptica ; 5, fovea hemispherical 6, common 

 opening of the superior and posterior semicircular 

 canals ; 7, opening of the aqueduct of the vestibule ; 

 8, opening of the aqueduct <>f the cochlea; '.'. the 

 scala vestihuli ; lo.seala tympani ; the lamina spiralis 

 separating 9 and 10. 



section, known respectively as the anterior or superior, the posterior, and the 



1 Rinne, quoted by Ilensen : Hermann'* Hindi, , ush ,/,,- Physiologie, 1880, Bd. iii. Tli. 'J, S. 26. 



2 Seashore: "Studies in Psychology," Bulletin University of Iowa, 1899. 



