SEC. 2. ON AUDITOKY SENSATIONS. 



619. 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 audi- 

 tory 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 repeti- 

 tion 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 intermediate stages. Much irregu- 

 larity 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 desir- 

 able to consider the two classes as distinct, and it will be con- 

 venient to deal first witli musical sounds. 



620. The sensations which are produced by musical sounds 

 possess three marked characters. In the first place our audi- 

 tory sensations, like our other sensations, may be more or less 

 intense ; and the character in a musical sound which corresponds 

 to the intensity of the sensation we call loudness. This is deter- 

 mined by the amplitude of the vibrations, by the amount of 



998 



