SEC 3. ON AUDITORY SENSATIONS. 



840. The vibrations which we call sound are transmitted as 

 we have seen to the perilymph through the fenestra ovalis, by 

 means of the tympanic membrane and chain of ossicles. The 

 vibrations of the perilymph in some way or other, by help of the 

 auditory epithelium, give rise in the fibres of the auditory nerve 

 to auditory impulses, and these reaching the brain are developed 

 into auditory sensations. Before we attempt to consider how the 

 vibrations of the perilymph thus give rise to auditory impulses it 

 will be convenient to adopt the plan which we pursued in the 

 case of vision and to deal first with some of the leading characters 

 of auditory sensations such as can be ascertained by psychological 

 methods. 



We readily recognize two classes of sensations ; the objective 

 causes of the one class we speak of as noises, those of the other 

 class as musical sounds. When we inquire into the physical 

 features of the two classes we find that the vibrations which 

 constitute a musical sound are repeated at regular intervals, and 

 thus possess a marked periodicity or rhythm. When no marked 

 periodicity is present in the vibrations, when the repetition of the 

 several vibrations is irregular, the sensation produced is that of a 

 noise. There is however no abrupt line between the two. Between 

 a pure and simple musical sound produced by a series of vibrations 

 each of which has exactly the same period, and a harsh noise in 

 which no consecutive vibrations are alike, there 'are numerous in- 

 termediate stages. Much irregularity may present itself in a 

 series of sounds called music, and in some of the roughest 

 noises the regular repetition of one or more vibrations may be 

 easily recognized. Still it will be desirable to consider the two 

 classes as distinct, and it will be convenient to deal first with 

 musical sounds. 



841. The sensations which are produced by musical sounds 

 possess three marked characters. In the first place our auditory 

 sensations like our other sensations, may be more or less intense ; 

 and the character in a musical sound which corresponds to the in- 

 tensity of the sensation we call loudness. This is determined by 



