HEARING 173 



from the bottom to the top of the spiral so that the fibres 

 become longer as the top is approached. The fibres have 

 been likened to the strings of a harp, and it has been sug- 

 gested that they are tuned to resonate to different notes so 

 that waves of different frequencies in the surrounding fluid 

 make the corresponding fibres vibrate. At the inner side of 

 the floor there is a ridge of cells running the length of the 

 tube under a little shelf that projects over it. The free ends 

 of the cells in the ridge are covered with minute "hairs" 

 pointing towards the overhanging shelf; the other ends are 

 surrounded with a network of nerve fibres. The hair cells 

 are really touch cells and send an impulse along their nerve 

 fibres if they touch the overhanging shelf. When waves of 

 vibration pass through the upper and lower tubes the fibres 

 in the floor of the middle tube that are tuned to those par- 

 ticular notes are thrown into vibration so that the hair cells 

 above them are bounced up and down and the hairs touch 

 the shelf above. At each touch they are stimulated, a physico- 

 chemical change takes place in the cell and a nerve impulse 

 is sent along the associated nerve fibre towards the brain. As 

 Professor Pumphrey has said, the inner ear is a frequency 

 analyser of great sensitivity, resolving power and range. 



This explanation of the way in which the ear works has 

 been challenged, and other theories which postulate that 

 the different frequencies received are sorted out in the brain 

 rather than the inner ear have been suggested. There is, 

 however, one anatomical feature that strongly supports the 

 explanation. The spiral tube is the only part of the body 

 that is the full adult size in the infant — it is nearly as large 

 in a new-born baby as in a full-grown man, and it scarcely 

 increases in size at all during the period of growing up. If 

 it was small in proportion to the rest of the body in a baby 

 we should expect children to be deaf to low notes and to be 

 able to hear notes far above those that an adult can hear. 

 To a slight extent this is true, for children can hear notes 

 somewhat above the normal range for adults, but in the 

 main the range of hearing in a small child is the same as 

 that in an adult. The reason for this is probably that the 

 resonant fibres in the inner ears of both are practically the 

 same length. 



