168 ON THE SENSES. 



plained nature, and probably essentially the same in all nerves, whetber it be 

 genemted by waves of light or of sound, by pres.?ure or heat, by objects of taste 

 or of smell. At the peripheral end of all the nerves of sense wlaich we have 

 heretofore considered, we have everywhere found or must suppose an appro- 

 priate terminal apparatus, whose function it is so to elaborate the excitation 

 which is received from without, that a movement of the current in the fibres of 

 the nerve shall ensue. This is the case with the nerves of hearing. It is 

 nearly certain that it is not the simple mechanical agitation of the filaments 

 of the auditory nerve which excites them, for this agitation excites no other 

 nerve, yet is the nerve of hearing as like to all others as one copper wire to 

 another ; the wave of sound must first call into existence some other action 

 which is capable of stimulating the nerve fibre. Of what nature it is, and how 

 the waves of the fluid of the labyrinth produce it, is wholly unexplained ; we 

 can only designate with certainty the seat and instruments of this enabling 

 action ; there are here as elsewhere, at the outer ends of the nerves, the minute 

 fibres and bulbs which the microscope discloses to us. Still less have we any 

 intimation how the " current" of the auditory nerves, generated mediately by the 

 undulatory movement, acts upon the brain and the soul, so that the latter forms 

 the specific conception of sound. We can here only refer to the general reason- 

 ing by which, in previous essays, it has been shown that it is the quality of th« 

 'in7ier central and terminal apparatus of the nerve fibres which determines the 

 characteristic effect of the current, the nature of the sensation. Who knows 

 whether, with the sounding plummet of experiment, we shall ever succeed in 

 exploring these deepest mysteries of existence? 



The final result of every excitation of the auditory nerve produced by a 

 movement of the kind we have been describing is a spiritual incident, which, 

 in general terms, we denominate a sensation of sound, but of which we distin- 

 guish several qualities. We contrast a sensation of tone with mere noise ; we 

 divide sounds according to their "pitch " and their tone; each of these quali- 

 ties again is susceptible of an endless gradation of intensity from the lightest, 

 only to be perceived by the most strained attention, to that which seems to 

 penetrate us by its intolerable force. The qualities, like the intensity of the 

 sensation, are determined by certain momenta referable to the nature of the ex- 

 ternal excitation ; thus the pitch of the sound depends on the number of the 

 waves which in a given time reach the ear ; the tone on the form of the waves ; 

 the intensity on the magnitude of the displacement which the wave communi- 

 cates to the particles. The explication of these relations is thus far exclusively 

 the affair of physics ; we shall not here enter at large into this branch of learn- 

 ing, because it is at present really not susceptible of a physiological interpreta- 

 tion ; that is to say, although we know from experience that a determinate, sub- 

 jectively recognized sound is causatively conditioned on certain properties of 

 the external movement which are very imperfectly understood, we are unable 

 to explain the why ; to specify how this or that form of movement acts upon 

 the ear ; how it operates upon the nerve ; much less can we describe the specific 

 quality of the nervous current, which is its result, or, in the last place, the 

 quality of the sensation. Referring the reader, therefore, to the popular expo- 

 sitions of writers on physics, we shall confine ourselves here to a few observa- 

 tions. A sound of some definite pitch arises when a sound-producing body — 

 a string, for instance— rcxecutes a certain number of vibrations in a unit of time, 

 so long as this number is not too small or too great. In other words, the vibra- 

 tions are only audible, only capable of producing the appropriate excitation of 

 tht- nerves, within certain limits of frequency. Under favorable circumstances we 

 can hear a sound — the deepest that is possible — when a string, an air-column, 

 &c., executes sixteen vibrations in a second, while with yet slower vibrations 

 the auditory nerve, under all circumstances, remains at rest, be the. intensity as 

 considerable as it may. On the other hand, there is also a maximum velocity 



