COORDINATION 



407 



of tightly stretched connective tissue fibers 

 which are longer at the top of the spiral and 

 shorter at the larger or lower end. Above 

 this membrane are the so-called hair cells 

 which send out nerve fibers that make up 

 a part of the auditory nerve. Overlying the 

 hair cells is the tectorial membrane, a thin 

 sheet of tissue, which lies very close to the 

 tiny hairs projecting from the hair cells. 

 There are other structures too which, how- 

 ever, need not be considered in this discus- 

 sion. How air vibrations are converted into 

 impulses that give the sensation called 

 hearing is the next problem. 



Sound perception. The fluid in the canals 

 of the cochlea is set into vibration by the 

 stirrup entering the oval window. It seems 

 probable, although no one has seen it, that 

 these vibrations set the basilar membrane 

 into sympathetic undulations which cause 

 the hair cells to touch the tectorial mem- 

 brane, and that such touch stimuli set up 

 impulses in the nerve fibers of the hair cells. 

 Since the basilar membrane is shorter and 

 tighter at the bottom, or largest, portion 

 of the cochlea (which seems quite con- 

 trary to reason), it probably responds to 

 the higher notes whereas the hair cells at 

 the top of the spiral are stimulated by the 

 low notes. This can be borne out be experi- 

 mentation. If an experimental animal is ex- 

 posed to sound of high pitch and consid- 

 erable amplitude (loudness) for a long 

 period of time, the hair cells in the lower 

 end of the cochlea are destroyed and the 

 animal is deaf over this range. Other seg- 

 ments of the organ of Corti can be de- 

 stroyed in a similar manner by appropriate 

 frequencies. The same phenomenon has 

 been observed in people suffering from the 

 so-called "Boiler Makers" disease where the 

 constant din of a trip hammer over many 

 years finally destroys that portion of the 

 organ which records the same pitch as that 

 of the hammer. It seems, then, that vibra- 

 tions coming in stimulate various parts of 

 the organ of Corti according to the fre- 

 quency of the vibration (pitch). When vi- 



brations of various pitches come in simul- 

 taneously the corresponding segments must 

 be stimulated. This is quite remarkable 

 when it is recalled how many different 

 tones can be distinguished when listening 

 to a symphony, for example. Certainly the 

 vertebrate ear is one of the most amazingly 

 complex organs found in any living thing. 



Hearing defects. The human ear can de- 

 tect low and high tones ranging from about 

 20 to 16,000 vibrations per second. A 

 young child can hear even greater ranges, 

 and some animals such as the dog and bat 

 can hear vibrations considerably beyond 

 that detected by the human ear. In man, the 

 range diminishes in the upper limits with 

 advancing years. 



Deafness is usually caused either by 

 faulty transmission of sound through the 

 middle ear or by some difficulty in the 

 cochlea, rarely by any deficiency in the cen- 

 tral nervous system. One of the common 

 causes of deafness is middle ear infections. 

 Infectious bacteria can make tlieir way up 

 the eustachian tube to the middle ear 

 where they can cause damage to the ear 

 bones or drum. Such infections can often 

 be cleared up by puncturing the eardrum 

 to allow the pus to drain out through the 

 ear canal, thus preventing severe damage 

 to the ear bones and drum. Repeated infec- 

 tions usually result in some impairment of 

 hearing. With the advent of antibiotics such 

 infections are not as common in children 

 as they once were and the coming genera- 

 tions should suffer less from deafness. 



With the refinement of electronic equip- 

 ment, hearing aids have gradually been 

 perfected to a point where they are very 

 useful to those who have imperfections in 

 the transmitting portion of their ears. Ob- 

 viously defects in the cochlea cannot be 

 compensated for by hearing aids. These in- 

 struments merely amplify the tones so that 

 sluggish or imperfect ear bones will, by 

 sheer force of the vibrations, pick up and 

 transmit them to the inner ear. The wear- 

 ing of hearing aids should cause one no 



