1 36 HUMAN PHYSIOLOGY 



and a descending branch. (1) The ascending branch sends 

 fibres on to the cerebrum (CB.\ and to the superior 

 vermis of the cerebellum (CBL.). These fibres give off 

 collaterals to the nucleus of Deiters (N. Deit.), from 

 the cells of which fibres pass, which divide, some run- 

 ning on the same side, some on the opposite side ; one 

 branch passing up to the oculo-motor mechanism (N.vi.), 

 the other passing down the spinal cord to send collaterals 

 to the cells in the grey matter. (2) The descending 

 branch forms connections with the medullary nuclei as it 

 passes down. 



Sound Perception. The qualities of sound which can 

 be distinguished by the sense of hearing, are loudness 

 amplitude of vibration ; pitch rate of vibration ; and 

 quality the character of the sound given by the over- 

 tones. The perception of this last is essentially a perception 

 of pitch. 



It is easy to understand how the peripheral neurons 

 in the internal ear are more powerfully stimulated by 

 the greater variations in the degree of pressure which 

 are produced by more powerful aerial waves, and how 

 the greater stimulation of the receptive centre in the 

 brain will be accompanied by a sensation of greater 

 loudness. 



A study of the structure of the cochlea seems to 

 show a mechanism well suited to afford a means of 

 estimating the pitch of a note and the existence of over- 

 tones. The fibres of the basilar membrane may be com- 

 pared to the strings of a piano, each one of which, or 

 each set of which, will be set in vibration by a particular 

 note. 



The power of distinguishing differences of pitch varies 

 in different individuals. It is more acute for notes of a 

 moderate rate of vibration, from 46 to 4000, than for very 

 slow or very fast vibrations. 



The range of perception of pitch also varies greatly, some 

 people hearing notes as low as 20 vibrations per second, 

 and others hearing them up to 40,000 per second. 



