PHYSIOLOGICAL 255 



each one corresponds to a particular note, as a violin string does; 

 and that when that note reaches the ear from outside, the appro- 

 priate fibre vibrates in sympathy, or by resonance, as it is said, and 

 that this sympathetic vibration causes impulses to be sent to the 

 brain and there interpreted. It is well known that if a note is soimded 

 beside a piano, the appropriate string of the piano will vibrate and 

 sound in sympathetic resonance. 



One of the most striking confirmations of this theory is given by 

 experiments in which guinea-pigs were exposed for weeks at a time 

 to the sounding of particular notes on an organ-pipe. It was then 

 found by examination that specific regions of the basilar mem- 

 brane had undergone degeneration, and that the region affected 

 varied with the note sounded. 



It is rather difiicult to compare the hearing organs of lower 

 animals with those of man. In mammals and birds the sense of 

 hearing is certainly well developed, sometimes better than in man, 

 so that vibrations over a wider range may be detected (the human 

 ear detects vibrations between 40 and 40,000 per second, roughly, 

 but cats and dogs are certainly sensitive to much shriller notes). 

 But in lower vertebrates and invertebrates it is not easy to be clear 

 about the definition of "hearing". If a fish is sensitive to vibrations 

 of similar frequency in the water, is that the same thing as "hearing" 

 vibrations in air? And if so, is the sensitiveness of an earthworm 

 to vibrations in the surrounding soil the same thing again, or is 

 this to be classed as a different sense ? The sense of hearing and the 

 sense of touch are closely allied, and it is hard to draw the line 

 between them and say. This shall be called hearing, and this not. 

 But in some insects at least an indubitable sense of hearing exists, 

 and it is interesting to notice that it is best proved in those species 

 in which one or both sexes has the power of producing some sound 

 at will. Insects have no true voices, but many can produce notes 

 by rubbing roughened parts of their hard cuticle together; in some 

 cases elaborate fiddle-like instruments have been evolved; the 

 trilling of the grasshopper is familiar to all. 



The Sense of Balance. — There are many different kinds of 

 animals that have ear-like organs and yet we have no evidence that 

 they can hear. In other cases where the ear-like organs are put 

 out of action, it makes no difference to the sense of hearing, if that 

 is present. What then is the meaning of the ear-like organs found in 

 myriads of backboneless animals? The answer is that they are 

 balancing or equilibrating organs, by means of which their possessors 

 automatically adjust their body so as to keep their balance in 

 swimming, flying, or running, or even their pose when resting. If 

 the organ is injured the animal will often tumble about anyhow 

 or swim on its back. A common type of an ear-like organ is a little 

 bag filled with fluid into which there project the fine hair-like pro- 



