103 2 THE SENSES 



ing of the movements of the auditory ossicles is thus secured. Another 

 theory is that the increased tension of the membrane renders it more 

 capable of responding to higher tones, and that the muscle thus acts as 

 a kind of accommodating mechanism. But Hensen has observed that 

 the tensor only contracts at the beginning of a sound, and relaxes again 

 when the sound is continued ; and this is difficult to reconcile with either 

 of these hypotheses. The muscle is normally excited reflexly through 

 the vibrations of the membrana tympani, but some individuals have 

 the power of throwing it into voluntary contraction, which is accom- 

 panied by a feeling of pressure in the ear, and a harsh sound. The 

 function of the stapedius 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. 



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. 



We have already recognized the organ of Corti, particularly the 

 hair-cells, as a sensory epithelium which constitutes the terminal 

 apparatus of the cochlear nerve. The adequate stimulus of the 

 auditory receptors is the periodic changes of pressure in the endo- 

 lymph. But there are various opinions as to how these vibrations 

 are transmitted to the hair-cells, and as to how the vibrations of 

 the hair-cells are translated into nerve impulses in the auditory 

 fibres. The pillars of Corti, the basilar membrane, and the mem- 

 brana tectoria, have in turn been regarded as the structures im- 

 mediately set into vibration by the changes in the endolymph. 

 The case for the tectorial membrane is perhaps the most plausible, 

 for its position renders it most capable of acting on the hairs. 

 Others have supposed that the hairs of the hair-cells are directly 

 affected by the endolymph. Some, despairing of further analysis, 

 content themselves with the conclusion that the organ of Corti 

 vibrates as a whole. Some of these theories will be again referred 

 to in considering what is the greatest problem of the physiology of 

 hearing, viz. : 



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 frequency of the aerial vibrations depends 

 also the pitch of a musical note. But there is this difference be- 

 tween 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 



