832 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



tions, analogous to visual successive contrast, by which our perception of every 

 sound is colored by the sounds which have preceded it. 



Imperfections of the Ear. Notwithstanding the mechanical provisions 

 for making the external and middle ear a perfect transmitting apparatus, 

 sound-perception is more or less modified by the action of the mechanism 

 under certain conditions. Thus, Helmholtz believed that various combina- 

 tional tones owe their origin chiefly to a periodic clicking in the joint between 

 the malleus and incus bones. The resonance of the ear is a familiar fact, 

 and through it high-pitched tones between e"" and g"" are reinforced and 

 heard with undue loudness. Certain hissing sounds, the chirp of a cricket or 

 the note of a locust, thus gain their intensity. This resonance probably is a 

 feature of the external auditory meatus, since it is at once destroyed by apply- 

 ing a small resonator to the ear (Helmholtz). 



Perception of Time Intervals. The ear is eminently the sense apparatus 

 for determining small intervals of time. Flashes of light succeeding each 

 other at the rate of twenty-four in a second are fused in a continuous luminous 

 impression by the eye, but by the ear at least one hundred and thirty-two audi- 

 tory impulses as beats may be heard separately in a second. The power which 

 the ear possesses of resolving complex air-waves into the host of pendular 

 vibrations which may enter into their formation finds no analogy in the eye 

 (Helmholtz). 



Musical Tones and Noises. The important feature of the physical 

 processes which give rise to musical tones is their periodicity. Every musical 

 tone is produced by a regular succession of alternate rarefactions and condensa- 

 tions in the air. The remaining class of sounds, known as noises, differs from 

 musical sounds in the respect that such sounds are produced by an irregular 

 succession of air-waves one in which the interval between phases of conden- 

 sation and rarefaction does not remain constant as in a musical note. Noises 

 are for the most part made up of short musical notes so associated as not to 

 " harmonize " with one another. As expressed by Helmholtz, the sensation 

 of a musical tone is due to a rapid periodic motion of a sonorous body ; the 

 sensation of a noise, to non-periodic motions. 



Functions of Different Parts of the Ear. Concerning the functions of 

 the diiFerent parts of the internal ear in their relation to sound-perception, it 

 is generally believed, as previously stated, that the basilar membrane of the 

 cochlea, with the nervous elements seated on it, is the organ concerned in the 

 reception and transmission of musical sounds. There are a sufficient number 

 of fibres in the basilar membrane to allow several to vibrate with every 

 audible tone. 



It cannot, however, too strongly be impressed that no theory of physiolog- 

 ical action should be accepted definitively without rigid experimental proof, and 

 such evidence concerning the definite functions of the cochlea is almost wholly 

 wanting. The sensory hair-cells on the maculaB of the saccule and the utricle 

 have been thought to have the duty of vibrating in response to any agitation 

 imparted to the perilymph, without regard to its periodic character; they 



