HEARING. 185 



gives it its quality. If, for instance, one hears middle C sounded on a violin it 

 has a quality different to that of the same note on the piano, and this again is 

 different from the sound of the tuning-fork, which, as we saw, gave a simple 

 pendular curve. The reason for this difference is that none of the sources in 

 the common instruments swing with a single vibration. A violin string, for 

 instance, swings as a whole, the rate of vibration of the whole length determining 

 the pitch of the lowest or fundamental tone in the composite curve of its note. 

 As well as this, however, the string tends to swing as if it were subdivided into 

 short lengths, halves, quarters, thirds, etc. (Fig. 53). The points dividing these 

 lengths swing with the vibration of the whole; the parts between swing with the 

 rhythm of the whole and with that of the part as well. The rate of vibration of 

 the divisions is to the rate of the whole inversely as the length of the division is 

 to the length of the whole. The result of this is that the sound wave arising from 

 the string is a composite one, like those which we have described, made up of a sine 

 curve with a period like that of the vibration of the whole string, upon which a 

 number of other waves have been superimposed, waves with rates in simple 

 arithmetical ratios (1/3, 1/4, 1/6, etc.) to the first. These arise from the vibra- 

 tions of the parts and are known as the OVERTONES. It is the number and in- 

 tensity of the. overtones which determine the quality of the note and it is because 

 these differ on different instruments that we are able to distinguish from one 

 another the notes that they give. 



THE TRANSMISSION OF SOUND WAVES IN THE EAR. 



Of the three parts of the ear, the external and middle chambers are only 

 concerned with the transmission of the sound waves, the essential organ of hearing 

 being entirely contained in the internal ear. The waves travel in air as far as the 

 inner end of the EXTERNAL AUDITORY MEATUS. Here the alternation of com- 

 pression and rarefication of which they are made up sets in motion the TYMPANIC 

 MEMBRANE. This membrane is made up of two epithelial layers, enclosing 

 between them a layer of connective tissue which contains both radial and circular 

 fibres. In shape it is somewhat peculiar, being slightly vaulted, with its convex 

 side inward. The apex of the vault, or UMBO, which forms a centre for the 

 radiating and circular fibres, is not exactly at the middle of the membrane but a 

 little above it. Because of its peculiar shape the membrane responds to waves 

 of a very large range of rates, instead of having a strongly marked natural period 

 of its own and being set in motion mainly by waves of this period, as would be the 

 case if it were simply a flat drum-head. 



THE MIDDLE EAR. Sound waves travelling in the middle ear are no longer 

 carried by air particles but by the swinging of a chain of three small bones, one 

 end of which is fastened to the end of the tympanic membrane and the other to 

 the membrane of the fenestra ovalis, one of the openings in the bony partition 

 between the middle and the inner chambers. The connection with the tympanic 

 membrane is through a long process, the manubrium of the MALLEUS, the first of 

 the three bones. The body of a malleus is connected by a joint with the second 



