io8 INTRODUCTION TO GENERAL PHYSIOLOGY 



place. The number of vibrations in what we call musical sounds 

 lies between about 40,000 and 30 per second. What we call the 

 loudness of a sound depends on the degree of changes in density, 

 or, what comes to the same thing, to the amplitude of the back- 

 ward and forward movement of the particles of the vibrating 

 substance, since the more they have congregated together at one 

 moment, the further have they come. There is another property of 

 sound, shown most markedly by the difference between the same 

 note played on the violin and on the flute. This is called quality, 

 and will be referred to presently. 



What the ear has to do, then, is to transform periodic changes 

 in density of the air into something of the nature of an actual 

 pressure or pull upon the endings of nerves, in such a way as to 

 stimulate them. When these air vibrations enter the ear, they 

 come against the " drum," a membrane stretched across the passage. 

 The membrane is caused to move in and out by the periodic 

 changes of pressure upon it. The important point is that it moves 

 equally well to any rate of vibration, on account of the fact that 

 it has no particular rate of its own, as an ordinary drum has. This 

 is partly due to the shape of the membrane and partly to the fact 

 that it is connected to a series of small bones which prevent its free 

 vibration. The result is that it follows exactly the smallest 

 changes in air pressure and passes the movement on to the end of 

 the chain of bones, unaltered in wave form, but, owing to the lever 

 action of the bones, .diminished in amplitude and correspondingly 

 increased in force. The further end of the bony lever is fixed to a 

 small membrane covering an aperture at the end of a canal in hard 

 bone. This canal has a spiral form, like a snail's shell, hence 

 called "cochlea," from the Latin name. It contains liquid, and on 

 this liquid a periodic series of pressures is exerted by the end of the 

 chain of bones. Suspended in the liquid is a complex structure in 

 which the auditory nerve ends, the " organ of Corti." The details 

 of this organ can only be given here in a general way, so far as 

 necessary to understand its mode of action. 



But, first of all, what is the nature of the vibrations set up in 

 the liquid by the periodic changes of pressure upon it? Let us 

 see what would happen supposing that the sound waves in the air 

 hit directly the end of such a column of liquid. We know that 

 they are transmitted, and may be transferred to air again at the 

 opposite end of the column. Sound can only be transmitted owing 

 to the elasticity and the compressibility of the material conducting 

 it. If this material were devoid of elasticity, the particles in 

 vibration would not return after being displaced, and if it were 

 incompressible, the alternate states of condensation and rarefaction 

 would be impossible. Although liquids are almost perfectly elastic 



