THE SENSE OF HEARING. 825 



cells, 3500, and of outer hair-cells in four rows, 12,000 ; outer rods of Corti, 

 5850 ; and inner rods of Corti, 5600. 



0. THE RELATION BETWEEN PHYSICAL AND PHYSIOLOGICAL SOUND. 



Production of Sound-waves. Sound, in its physiological meaning, is a 

 sensation which is the conscious appreciation of internal changes occurring in 

 certain cells of the cerebral cortex. Fibres of the auditory nerve come into 

 close relation with these cells, and in whatever way those fibres are excited 

 the result is one and the same, a sensation of sound. 



The elaborate apparatus of the middle and internal ear is so constructed 

 that the energy of mechanical oscillations in the external air is transmitted to 

 the terminations of the auditory nerves in a manner to excite them. 



Sound, in a physical sense, consists in waves of alternate condensation and 

 rarefaction travelling in the air from the point of origin of the sound, much as 

 waves radiate over the surface of water from the point where a stone is dropped. 

 Any sudden impulse, such as a puff of air, or the vibration of a solid body, 

 as a stretched string or a tuning-fork, pushes the adjacent molecules of air 

 against those further removed, and this impulse produces an area, or aerial 

 shell, of increased density or condensation. The air being perfectly elastic, 

 the molecules, relieved from pressure, spring back even beyond the position 

 of equilibrium, and leave an area of decreased density or rarefaction. Thus 

 a wave, consisting of a shell of condensation succeeded by a shell of corre- 

 sponding rarefaction, moves through the air. This single air-wave is the 

 simplest element of physical sound. When a number, no matter how great, 

 of sound-waves simultaneously excite the same particle of air, the resultant 

 motion of that particle is the algebraic sum of all the motions imparted to it 

 by the single sound-waves considered separately. As any elastic body, when 

 set vibrating, continues its oscillations for a time, so is it probable that strictly 

 isolated air-waves do not occur. Any elastic body, such as a stretched string, 

 -or a tuning-fork, when set in vibration, sends out from itself a series of air- 

 waves which succeed one another at a rate identical with the rate of vibration 

 of the elastic body. Such a regular succession of air-waves striking upon the 

 tympanic membrane sets the latter into correspondingly regular oscillations 

 and produces in the auditory apparatus the sensation of musical tone. 



Loudness and Musical Pitch. The more vigorous the vibrations of the 

 oscillating body, the more forcibly are the air-molecules which are struck by it 

 driven forward ; and the greater their excursion or amplitude of movement, 

 the greater is the force with which the tympanic membrane is driven inward 

 when the moving air-wave strikes it. The loudness of the tone manifestly 

 depends upon the extent of motion of the tympanic membrane, as does this on 

 the amplitude of air-motion. Different elastic bodies have different natural 

 rates of oscillation. The more rapid the rate, the more frequent is the succes- 

 sion of air-waves that strike upon the ear. Musical pitch is determined by 

 the number of air-waves which pass a given point in a unit of time, or, in 

 other words, by the rate of vibration of the sound-producing body. When 



