

SENSE OP HEARING. 665 



development of the cochlea. In fact, the cochlea of a singing bird 

 resembles that of a crocodile or a serpent more closely than that of a 

 quadruped or a man. At the same time, the other parts of the internal 

 and middle ear in birds, the double sac of the vestibular cavity, the 

 membranous semicircular canals and ampullae, the fenestra ovalis and 

 rotunda, the chain of bones and the ineinbrana tympani, are all highly 

 developed ; some of them nearly or quite as much so as in the mamma- 

 lian class. These facts throw a certain degree of doubt upon the special 

 office of the cochlea in the perception of auditory sensations. 



Persistence of Auditory Impressions and the Production of Musical 

 Notes. The sensation excited by a sonorous vibration continues for a 

 short time after the cessation of its cause. Usually the interval be- 

 tween successive impulses is more than sufficient to allow the continued 

 impression to disappear, and the ear distinguishes without difficulty the 

 succession of sounds. But if the impulses follow each other at equal 

 intervals, and with a certain degree of rapidity, they produce upon the 

 ear the impression of a continuous sound, and this sound has a higher 

 or lower pitch according to the rapidity with which the vibrations are 

 repeated. The numerical relation of different musical notes thus pro- 

 duced has been studied by means of various instruments. One of these 

 is the siren of Savart, in which successive puffs of air are emitted from 

 the body of the machine through small openings, with a degree of 

 rapidity which can be varied at will and registered by an index attached 

 to the moving parts. Another method is that in which the shocks are 

 given by the points of a toothed wheel turning with known velocity, 

 and striking, in their passage, against the projecting edge of a card. 

 In another modification of the same plan, the revolving wheel carries 

 one or more projecting rods, which pass, in succession, through a cor- 

 responding slit in a stiff board ; making at each transit an atmospheric 

 concussion, owing to the instantaneous displacement and rebound of 

 the air at the opening. Finally, the number of vibrations correspond- 

 ing to a particular note may be registered by attaching to the extremity 

 of a diapason, or tuning-fork, a light stilet which traces upon the 

 blackened surface of a cylinder, revolving at a known rate, an undu- 

 lating line (Fig. 146, a) ; the number of undulations within a given 

 space indicating the frequency of the vibrations of the tuning-fork. A 

 simple vibration represents the single oscillation of a solid body, or the 

 particles of a fluid, in one direction ; a double vibration is the complete 

 to-and-fro movement of a particle, which brings it back to its original 

 position. 



By this means it is found that sonorous impulses, which follow each 

 other with a rapidity of less than sixteen times per second, are readily 

 distinguishable as separate shocks ; but above that degree of frequency 

 they become merged into each other, and produce the sensation of a 

 continuous sound. In case the repetition of the shocks takes place 

 at irregular or unequal intervals, the only characters perceptible in 

 the sound are its intensity and the peculiarities due to the special 

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