SENSE OF I1EAKING LAWS OF PROPAGATION OF SOUND. 785 



iiate in hair-cells (see Fig. 278). The number of the arches of Corti is 

 estimated by Kolliker at 3000, or about 33 to each half note of the ordi- 

 narily audible seven octaves. 



642. In order to gain any definite idea of the uses of different parts of 

 the Ear, it is necessary to bear in mind that sounds may be propagated 

 amongst solid or fluid bodies in three ways: by reciprocation, by resonance, 

 and by conduction. 1. Vibrations of reciprocation are excited in a sound- 

 ing body, when it is capable of yielding a musical tone of definite pitch, 

 and another body of the same pitch is made to sound near it. Thus if two 

 strings of the same length and tension be placed alongside of each other, 

 and one of them be sounded with a violin bow, the other will be thrown into 

 reciprocal vibration ; or if the same tone be produced near the string in any 

 other manner, as by a flute or a tuning-fork, the same effect will result. 2. 

 Vibrations of resonance are of somewhat the same character ; but they occur 

 when a sounding body is placed in connection with any other, of which one 

 or more parts may be thrown into reciprocal vibration ; even though the 

 tone of the whole be different, or it be not capable of producing a definite 

 tone at all. This is the case, for example, when a tuning-fork in vibration 

 is placed upon a sound-board ; for even though the whole board have no defi- 

 nite fundamental note, it will divide itself into a number of parts, which will 

 reciprocate the original sound, so as greatly to increase its intensity ; and the 

 same sound-board will act equally well for tuning-forks of several different 

 degrees of pitch. When a smaller body is used for resonance, however, it 

 is essential that there should be a relation between its fundamental note and 

 that of the sonorous body ; otherwise no distinct resonance is produced. 

 Thus, if a tuning-fork in vibration be held over a column of air in a tube, 

 of such a length that the same note would be given by its vibration, its 

 sound will be reciprocated. And if it be held over a pipe, the column of 

 air in which is a multiple of this, the column will divide itself into that 

 number of shorter parts, each of which will reciprocate the original sound, 

 and the total action will be one of resonance. But if the length of the pipe 

 bear no such correspondence with the note sounded by the tuning-fork, no 

 resonance is given by the column of air it contains. 3. Vibrations of con- 

 duction are the only ones by which sounds can strictly be said to be propa- 

 gated : these are distinguishable into various kinds, into which it is not 

 requisite here to inquire. It should be remarked, however, that all media, 

 fluid, liquid, or solid, are capable of transmitting sound in this manner; a 

 vacuum being the only space through which it cannot pass. The transmis- 

 sion is usually much more rapid through solid bodies than through liquid; 

 and through liquid than through gaseous. The greatest diminution in the 

 intensity of sound is usually perceived when a change takes place in the 

 medium through which it is propagated, especially from the aeriform to the 

 liquid. 



643. The detailed application of these principles has been most elaborately 

 worked out by Professor Miiller; and the following statement is little more 

 than an abstract of the results of his experimental investigations; of which 

 the first series bears specially on the case of those animals, which, living 

 immersed in water, receive the sonorous undulations through that medium. 

 The labyrinth of such as possess a distinct organ of hearing, is either entirely 

 inclosed within the bones of the head, as in the Cephalopoda and in the Cy- 

 clostome and Osseous Fishes; or, its cavity being prolonged to the surface 

 of the body, it is there brought into communication with the conducting me- 

 dium by means of a membrane, besides receiving the vibrations through the 

 medium of tue solids of the body, as is the case in Cartilaginous Fishes and 

 Crustacea: I. Sonorous vibrations, excited in water, are imparted with con- 



