THE ORGANS OF SENSE. 147 



be necessary here to state the known facts and the view, based upon 

 these, which is most generally held. 



(1) It is a well-known physical fact that a string which vibrates 

 with the rhythm of, say, middle C, will be thrown into vibration if that 

 note is sounded near it. Similarly, if a vibrating tuning fork be held 

 over the mouth of a tall glass jar and water be slowly poured into the 

 jar, when the water is at a certain depth the sound of the tuning fork 

 will be intensified by the resonance of the column of air in the jar. 



(2) From the distribution of the cochlear nerve fibres around the 

 hair cells, it may be assumed that the latter are the end-organs for 

 hearing. 



(3) The rods of Corti are not present in the cochlea of birds, and 

 are therefore not an essential part of the auditory mechanism. 



(4) The basilar membrane is composed of about 24,000 radial fibres, 

 and it increases in width from the base to the apex of the cochlea, the 

 shortest fibres being 0'041 mm. and the longest 0'495 mm. in length. 



(5) Experimental destruction of the base of the cochlea in dogs 

 made the animals deaf to high notes, whereas destruction of the apex 

 resulted in deafness to low notes. Similar results have followed disease 

 in man. 



These facts suggest that different parts of the basilar membrane 

 resonate to notes of different pitch, the longer fibres responding to low 

 notes and the shorter fibres to high notes. The vibrations of the basilar 

 membrane set up waves in the endolymph, by which the hair cells are 

 stimulated, after-vibrations being damped by the tectorial membrane. 

 On this theory, the analysis of sound takes place in the cochlea, each note 

 causing definite fibres of the basilar membrane to resonate, and thus 

 acting as a stimulus to the hair cells opposite that part of the membrane. 

 If this hypothesis is correct, a note of any particular pitch will always 

 excite an impulse in the same nerve fibres. 



The auditory impulses reach the cerebral cortex of the temporal lobe 

 by the auditory tract (p. 91), arriving first at the audito-sensory area, 

 and being transferred to the audito-psychic area. The further convey- 

 ance of the impulses to the association areas enables judgments to be 

 arrived at as to the nature of the sounds, as, for example, the rumbling 

 of thunder or the meaning of spoken language. 



In some of the lower animals the judgment of the direction from 

 which a sound proceeds is aided by movement of the external ears. In 

 man the projection of sound is more difficult, especially if its source is 

 in line with the mesial plane of the body. Some assistance is obtained 

 by moving the head and noting to which ear the sounc| is more distinct 

 in each position. As a rule, man relies largely on the co-operation of sight 



