772 



THE SENSE OF HEARING 



the fenestra ovalis. From here these oscillations spread throughout 

 the perilymph of the vestibule and pass toward the semicircular canals 

 as well as toward the cochlea. In most cases, however, they fail 

 absolutely in activating the sense-organs of equilibrium in the utricle 

 and ampullae of these canals, because the latter do not lie in the direct 

 course of these waves, and are not specifically adapted to them. The 

 cochlea, on the other hand, turns its funnel-shaped basal portion 

 directly toward the vestibule and into the path of these oscillations. 



Besides, this structure gives lodg- 

 ment to the sense-organ which is 

 specifically set aside for their recep- 

 tion. 



The Osseous Canal of the 

 Cochlea. — The central chamber of 

 the labyrinth, or vestibule, measures 

 5 mm. in diameter and communicates 

 anteriorly with the cochlea. The 

 latter is a cone-shaped structure, 

 ^^^^^^^^ measuring 9 mm. across at its base, 



^ NIk^ ^^^^^VEfl^B. ^^^ ^ "^™* ^^^^ ^^^ ^^^^ ^^ ^^^ apex. 

 15 LLoIIIIIIIn ji^^^^^K iI[T\ m The tip or cupola of the latter is di- 



rected outward and slightly forward 



V^^^^^^^yMjII^^ and downward. It contains a canal 



£ ^^^^Lr /^™^^^ \^^^B which is twisted upon itself two and 



^^HBHp||^H|^^ one-half times in the manner of the 



shell of a snail. This canal measures 

 Fig. 388.-DIAGRAMMATIC View OF THE ^^^^^ 33 ^^ ^^ j^^^j^^ j^ .^ 

 Internal kjAR. . *p 



1. Tympanic cavity; 2, Eustachian largest at its base, where it tueasures 



tube; 3, incus; 4, stapes; 5, vestibule of about 2 mm. in diameter. The cen- 



the internal ear (perilymph); 6 utricle; ^^^1 ^Ore arOUnd which it is WOUnd, 

 7, central canal of the cochlea; 8, scala . , , ,. , mi 



vestibuli; 9, saccule; 10, endolymphatic IS known aS the modlOluS. ihe 



duct between saccule and utricle; 11, latter COnsists of a central Spongy 



ampulla of semicircular canal; 12, canalis portion which is pierced by a tube 



reuniens; 13, scala tympam; 14, helico- ^ .,, ., . n . ■, r i 



trema; 15, fenestra ovalis. • With its VariOUS Collaterals for the 



reception of blood-vessels and the 

 fibers of the cochlear branch of the auditory nerve. A bony plate, 

 the lamina spiralis, projects from this central mass of bone almost 

 horizontally into the lumen of the cochlear canal, winding round into 

 its tip in the manner of a circular staircase. It partially divides the 

 lumen of this canal into two compartments or scalae; this division be- 

 ing made complete by a membranous septum which stretches straight 

 across from the end of the bony lamina to the opposite wall of the 

 canal. This is. the so-called basilar membrane. Below the latter, we 

 have the scala tympani and above it, the scala vestibuh. The cochlear 

 canal as a whole is placed in such a way that its vestibular scala faces 

 the foramen ovale, while its tympanic scala is directed toward the 

 foramen rotundum. These tubes communicate with one another 



