844 A MANUAL OF PHYSIOLOGY 



is unknown. Its contraction would tend to press the posterior end of 

 the foot-plate of the stapes deeper into the foramen ovale, and cause 

 the anterior end to move in the opposite direction ; but it is not easy 

 to see how this would affect the action of the auditory mechanism. 

 It has been suggested that the role of the stapedius is to damp the 

 oscillations of the stapes and orbicular ligament when very loud 

 sounds are listened to, and thus prevent shocks of too great intensity 

 from being transmitted to the labyrinth. 



The tensor tympani is supplied by the fifth nerve through a branch 

 from the otic ganglion; the stapedius is supplied by the seventh. 

 Paralysis of the fifth nerve may be accompanied with difficulty of 

 hearing, especially for faint sounds. When the seventh nerve is 

 paralyzed, increased sensitiveness to loud sounds has been observed. 



The Perception of Pitch Analysis of Complex Sounds. 

 As the eye, or, rather, the retina plus the brain, can perceive 

 colour, so the labyrinth plus the brain can perceive pitch. 

 The colour-sensation produced by ethereal waves of definite 

 frequency depends on that frequency; and upon the fre- 

 quency of the aerial vibrations depends also the pitch of a 

 musical note. But there is this difference between the eye 

 and the ear : that while the sensation produced by a mixture 

 of rays of light of different wave-length is always a simple 

 sensation that is, a sensation which we do not perceive to 

 be built up of a number of sensations, which, in other words, 

 we do not analyze the ear can perceive at the same time, 

 and distinguish from each other, the components of a com- 

 plex sound. When a number of notes of different pitch are 

 sounded together at the same distance from the ear, the 

 disturbance which reaches the membrana tympani is the 

 physical resultant of all the disturbances produced by the 

 individual notes, and it strikes upon the membrane as a 

 single wave. The ear or brain must, therefore, possess the 

 power of resolving the complex vibrations into their con- 

 stituents, else we should have a mixed or blended sensation, 

 and not a sensation in which it is possible to distinguish the 

 constituents of which it is made up. Two chief hypotheses 

 have been proposed to explain this physiological analysis of 

 sound: (i) the theory that the analysis takes place in the 

 labyrinth ; (2) the theory that it takes place in the brain. 



(i) Helmholtz attempted to explain the perception of 

 pitch on the assumption that in the internal ear there exists 

 a series of resonators, each of which is fitted to respond by 



