780 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



so simple that it may be questioned whether they can be said to possess an 

 organ of hearing, if by this term we imply anything more than the mere 

 consciousness of sonorous vibrations. There is a considerable difference, 

 however, between the Eye and the Ear, in regard to the special purposes 

 for which they are respectively adapted. In the former we have seen that 

 the whole object of the instrument is to direct the rays of light received by 

 it, in such a manner as to occasion them to fall upon the expansion of the 

 optic nerve in similar relative positions, and with corresponding proportional 

 intensities, to those which they possessed when issuing from the object. We 

 have no reason to believe anything of this kind to be the purpose of the 

 Ear ; indeed it would be inconsistent with the laws of the propagation of 

 sound. Sonorous vibrations having the most various directions, and the 

 most unequal rates of succession, are transmitted by all media without modi- 

 fication, however numerous their lines of intersection ; and wherever these 

 undulations fall upon the auditory nerve, they must cause the sensation of 

 corresponding sounds. Still it is probable that some portions of the complex 

 organ of hearing, in Man and in the higher animals, are more adapted than 

 others to receive impressions of a particular character; and that thus we 

 may be especially informed of the direction of a sound by one part of the 

 organ, of its musical tone by another, and some other of its qualities by a 

 third. 



639. A single impulse communicated to the Auditory nerve through an 

 appropriate apparatus, seems to be sufficient to excite the momentary sensa- 

 tion of sound; but most frequently a series of such impulses is concerned, 

 there being but few sounds which do not partake, in a greater or less degree, 

 of the character of a tone. Any continuous sound or tone is dependent 

 upon a succession of impulses; and its acuteness or depth is governed by 

 the rapidity with which these succeed one another. It is not difficult to as- 

 certain by experiment, what number of such impulses or undulations are 

 required to give every tone which the ear can appreciate. Thus if, as in the 

 instrument termed the Syren, a circular plate with a number of apertures at 

 regular intervals be made to revolve over the top of a pipe through which 

 air is propelled, a succession of short puffs will be allowed to issue from this; 

 and, if the revolution be sufficiently rapid, these impulses will unite into a 

 definite tone. In the same manner, if a spring be fixed near the edge of a 

 revolving toothed wheel, in such a manner as to be caught by every tooth 

 as it passes, a succession of clicks will be heard ; and these too, if the revo- 

 lution of the wheel be sufficiently rapid, will produce a tone. The number 

 of apertures in the plate which pass the orifice of the pipe in a given time, 

 or the number of teeth which pass the spring, being known, it is easy to see 

 that this must be the number of impulses required to produce the given tone. 

 Each impulse produces a double vibration, forwards and backwards (as is 

 seen when a string is put in vibration, by pulling it out of the straight line) ; 

 hence the number of single vibrations is always double that of the impulses. 

 The maximum and minimum of the intervals of successive pulses, still ap- 

 preciable by the ear as determinate musical sounds, have been determined 

 by M. Despretz. 1 According to this observer the number of complete vibra- 



1 Comptes Rendus, torn, xx, p 1214. Hermann, Grundriss dor Physiologic, 1807, 

 gives the limits iit 40 and 1(5,000 double vibrations; Ilelmholtz at 10 and 38,000 

 single vibrations, or about 11 octaves. Blake (Boston Med. and Surg. Journ., 1872, 

 vol. x, No. 'j(t) found that children of 12-13 years of age could perceive a note pro- 

 duced by 40, '.iilO single vibrations at a distance of 34 feet. Young persons of 18-20 

 \ ears the same only at a distance of 13-16 feet, but a note produced by 36,864 simple 

 vibrations at 34 feet. Adults of 28-30 years could only hear at 34 feet a note pro- 

 duced by 32,708 vibrations. The deepest tone of orchestral instruments is the E of 



