616 THE NERVOUS SYSTEM. 



into two groups one going to the inner hair-cells, the other to 

 the outer. Schafer says that " after traversing the spiral lamina 

 they emerge in bundles, and the fibers then, having lost their 

 medullary sheath, pass into the epithelium of the inner hair-cell 

 region. Here some of them course at right angles, and are 

 directly applied to the inner hair-cells, whilst others cross the 

 tunnel of Corti, to become applied in like manner to the outer 

 hair-cells and the cells of Deiters ; but there does not appear to 

 be any continuity between the nerve-fibrils and the cell-substance." 



Vestibular Branch of Auditory Nerve. This is often spoken of 

 as the vestibular nerve. It divides into three branches : 1, supe- 

 rior, which is distributed to the utricle and the ampullae of the 

 external and superior semicircular canal ; 2, middle, which is dis- 

 tributed to the saccule ; and 3, inferior, which is distributed to 

 the ampulla of the posterior semicircular canal. 



Physiology of Hearing. Sound is defined by the Standard 

 Dictionary as : "1. The sensation produced through the organs of 

 hearing. 2. The physical cause of this sensation ; waves of alter- 

 nate condensation and rarefaction passing through an elastic body, 

 whether solid, liquid, or gaseous, but especially through the atmo- 

 sphere." 



Bodies which emit sound are called sonorous bodies, and are at 

 the time in a state of vibration. Thus a tuning-fork when set in 

 vibration produces in the air around it a series of condensations 

 and rarefactions which form " concentric spherical shells of air of 

 different densities. Each air particle swings to and fro in a very 

 short path along the radius of the sphere that is, the vibrations 

 are longitudinal " (Carhart and Chute). 



These waves are sound-waves, and when they enter the external 

 auditory meatus they set up vibrations in the membrana tympani. 

 . Through the ossicles these vibrations are transmitted to the peri- 

 lymph. The base of the stapes, which, with its annular membrane, 

 closes the fenestra ovalis, communicates these vibrations to the 

 perilymph in the scala vestibuli, along which waves of the fluid 

 travel, passing through the helicotrema and down the scala 

 tympani to the fenestra rotunda, the membrana tympani secun- 

 daria being pressed out as each wave reaches it. In this course 

 the waves of perilymph pass over the membrane of Reissner as 

 they travel up the scala vestibuli, and under the membrana 

 basilaris as they return down the scala tympani, thus setting up 

 corresponding vibrations in the endolymph which fills the membra- 

 nous cochlea. 



The external ear collects the sound-waves, and in some animals, 

 as in the horse, is very useful in determining the direction from 

 which sounds come ; these animals, by virtue of muscles attached 

 to the ear, can move it in all directions. In man such muscles 

 exist, but in a relatively undeveloped form, and are not under the 



