D THE EAR AND THE SENSE OF HEARING 227 



is not confined to mere patches, but is continued along the 

 whole length of the spiral. A branch of the auditory nerve 

 running up the axis of the spiral sends off, all the way up, 

 fibres, which, running in the basilar membrane, become con- 

 nected with the auditor)- epithelium along its whole length. 

 The auditor)- epithelial cells are arranged on the basilar mem- 

 brane all the way from the bottom to the top of the spiral 

 in a remarkable manner, and the structure so formed is spoken 

 of as the organ of Corti. In a section across a coil of the 

 cochlea, at about the middle of the basilar membrane, may be 

 seen a pair of rod-like cells called the rods of Corti, propped 

 up one against the other, so as to form an arch. These 

 pairs of rods occur one after the other all along the tube of 

 the cochlea, there being about 5000 pairs altogether. On the 

 outer side of the rods of Corti are several rows of epithelial 

 cells bearing short hair-like processes, and there is one row of 

 similar cells on the inner side of the rods. They are called 

 the outer and inner hair cells. The fibres of the auditory 

 nerves can be traced up to and are in connection with these 

 hair cells. 



Sound 



Anything causing a sound either vibrates bodily itself or 

 some of its particles vibrate, and as a consequence the air 

 around it is thrown into waves ; these waves, if of sufficient in- 

 tensity and frequency, produce, on reaching the ear, the sensa- 

 tion which we call sound. 



Sound travels through the air at the rate of iioofeeta 

 second. It is also transmitted through liquids and solids. 

 A musical sound or note is produced when the vibrations 

 follow each other regularly, that is, when similar vibrations 

 are repeated so many times a second ; when the vibrations 

 are irregular the sound is called a " noise." A musical sound 

 or a noise may be loud or feeble ; this depends on the strength 

 with which the vibrations affect the ear. A musical sound 

 has what is called a " pitch," low or high ; this depends on 

 the rapidity with which the vibrations are repeated ; when 

 they are repeated slowly the pitch is low, when rapidly high. 

 Besides pitch and loudness a musical sound has what is called 



