354 The Outline of Science 



alter with changes of pressure outside, is regulated by a channel 

 (the Eustachian tube) running to it from the roof of the mouth. 

 Three little bones (the hammer, anvil, and stirrup) convey 

 the vibrations of the drum to another drum, which is stretched 

 across the entrance to the real ear inside the skull. As the waves of 

 sound impinge on the tympanum and set it vibrating, the three 

 little bones work together and repeat the vibrations on the second 

 drum, the "oval window." Beyond this is a coiled shell which 

 contains the real organ of hearing a large number of hair-cells 

 (the "organs of Corti") interlacing with the fine fibres of the 

 auditory nerve. The vibration of the "oval window" agitates the 

 fluid inside this organ, and the hair-cells communicate this move- 

 ment to the nerves, which then convey it to the brain. Once more 

 we have a mechanism full of ingenuity in every part, and brief 

 descriptions of this kind are almost unjust to the various organs 

 of our body ; but to-day we should require a large volume to give 

 an account of our knowledge of the brain and sense-organs alone. 

 We have referred to the three small bones of the ear by which the 

 waves of sound are conveyed to the inner ear. "The history of 

 these bones is strange. The hammer was at an early stage of the 

 evolution of mammals a part of the lower jaw; the anvil was the 

 bone on the base of the skull, with which it articulated. When 

 mastication and molar teeth were evolved in the ancestry of mam- 

 mals, a new joint was formed in the lower jaw, and these two 

 bones the hammer and anvil were taken into the service of the 

 ear." 1 



11 

 THE DISCOVERY OF HORMONES 



Remarkable Discoveries about the Glands 



A physiologist would class the different parts of the body 

 as bones, muscles, nerves, and glands, and we have in conclusion 



1 Sir Arthur Keith, The Human Body. 



