CHAP, in.] HEARING, SMELL, AND TASTE. 559 



The stapedius muscle is supposed to regulate the movements 

 of the stapes, and especially to prevent its base being driven too far 

 into the fenestra ovalis during large or sudden movements of the 

 membrana tympani. It is governed by fibres from the facial 

 nerve. 



The Eustachian tube. This serves to maintain an equi- 

 librium of pressure between the external air and that within the 

 tympanum, and to serve as an exit for the secretions of that cavity. 

 Were the tympanum permanently closed the vibrations of the 

 membrana tympani would be injuriously affected by variations of 

 pressure occurring either inside or outside. The Eustachian tube 

 is undoubtedly open during swallowing, but it is still disputed 

 whether it remains permanently open, or is opened only at in- 

 tervals; probably it is, at most times, neither widely open nor 

 closely shut. 



Auditory Sensations. 



Each vibration communicated by the stapes to the perilymph 

 travels as a wave over the vestibule, the semicircular canals, and 

 other parts of the labyrinth; and from the perilymph is transmitted 

 through the membranous walls to the endolymph. From the 

 vestibule it passes on into the scala vestibuli of the cochlea, and 

 descending the scala tympani, ends as an impulse against the 

 membrane of the fenestra rotunda. In the regions of the maculae 

 and cristae the vibrations of the endolymph are supposed to throw 

 into corresponding vibrations the so-called auditory hairs. In the 

 cochlea the vibrations of the perilymph are supposed to throw into 

 vibrations the basilar membrane with the superimposed organ of 

 Corti, consisting of the rods of Corti with the inner and outer hair- 

 cells. The vibrations thus transmitted to these structures give rise 

 to nervous impulses in the terminations of the auditory nerves, and 

 these impulses reaching certain parts of the brain produce what we 

 call auditory sensations. We are accustomed to divide our auditory 

 sensations into those caused by noises and those caused by musical 

 sounds. It is the characteristic of the latter that the vibrations 

 which constitute them are periodical; they occur and recur at regular 

 intervals. When no marked periodicity is present in the vibrations, 

 when the repetition of the several vibrations is irregular, or the 

 period so complex as not to be readily appreciated, the sensation 

 produced is that of a noise. There is however no abrupt line be- 

 tween the two. Between a pure and simple musical sound pro- 

 duced by a series of vibrations each of which has exactly the same 

 wave-length, and a harsh noise in which no consecutive vibrations 

 may be alike, there are numerous intermediate stages. 



In both noises and musical sounds we recognise a character 

 which we call loudness. This is determined by the amplitude of 



