752 SPECIAL SENSES 



a physical impossibility for the different rods to vibrate individually, and 

 he regarded it as improbable that the rods are tuned in accord with 

 different musical notes. The present idea is that strands of the mem- 

 brana basilaris, stretched between the lower extremities of the pillars of 

 Corti, being of different lengths, vibrate in obedience to the waves of 

 sound conveyed to them through the auditory nerves ; but even this does 

 not afford an entirely satisfactory explanation of the mechanism of the 

 final appreciation of pitch. It is the fact, however, that disease of the 

 lower part of cochlea is attended with deafness to high tones, and 

 disease affecting the summit produces deafness to low tones. 



It is not necessary that sonorous vibrations should pass to the 

 cochlea through the external ear and parts in the middle ear. Sounds 

 may be conducted to the auditory nerves through the bones of the head 

 or through the Eustachian tubes, as is shown by the simple and familiar 

 experiment of placing a tuning-folk in contact with the head or between 

 the teeth, the ears being closed. 



The action of the two ears does not seem to be essential to the correct 

 appreciation of auditory impressions ; but variations in the force of such 

 impressions made on either ear aid in determining the direction of sounds, 

 although errors in regard to this often occur. 



The estimate of the distance of sounds is made by judging of their 

 intensity, in connection with information obtained through other senses, 

 especially the sense of sight. The power of estimating distance is 

 largely influenced by experience and education. 



Centres for Audition. The centres for audition in dogs and monkeys 

 are in the superior temporo-sphenoidal convolution (Ferrier, Munk). In 

 man these centres are in the first and second temporal convolutions of 

 the temporo-sphenoidal lobe, parts supplied by the fourth branch of the 

 middle cerebral artery. This has been ascertained by pathological ob- 

 servations as well as by experiments on the lower animals. In man the 

 action of these centres is not completely crossed, and destruction of the 

 centre upon one side does not cause total deafness in either ear. Com- 

 plete destruction of the centres on both sides, however, produces deaf- 

 ness in both ears. Injury of the first temporal convolution often is 

 followed by the condition known as word-deafness, in which the subject 

 hears the sound of words, but these sounds convey no idea. This is the 

 psychical auditory centre and is confined to the first temporal convolu- 

 tion on the left side (Wernicke). Word-deafness is analogous to the 

 condition already described under the name of word-blindness, and the 

 centre usually is confined to the left side of the cerebrum. It has been 

 suggested by Westphal that this centre may be on the right side of the 

 cerebrum in left-handed persons. 



