414 THE ORGANS OF SPECIAL SENSE 



covering the rods of Corti are bathed by a clear fluid, 

 the endolymph. This fluid comes from the sub- 

 arachnoid lymph spaces at the base of the brain. 



The hair cells resting on the organ of Corti are 

 practically end-organs of the cochlear branch of the 

 auditory nerve. Just how the filaments of this nerve 

 come in direct contact with these cells and develop 

 nerve impulses is not definitely understood. 



The Physiology of the Structures Concerned in 

 the Sense of Hearing. The sense of hearing is based 

 on the functions possessed by the structures within the 

 three portions of the ear, which receive and transmit 

 atmospheric vibrations set up in the external world 

 about us, to the sensitive hair cells of the organ of 

 Corti, where they are taken up and carried back by 

 the fibers of the auditory nerve to the centres in the 

 cerebrum and the brain becomes conscious of the 

 sensations of sound. 



Stimuli. All stimuli which produce the sense of 

 sound must be in a state of motion, and thus create 

 vibrations which are communicated to the air in 

 which they are moving to and for, setting it into 

 waves, called sound waves. These sound waves in 

 turn reach the tympanic membrane through the 

 external auditory canal, and set it into vibration; 

 then they are transmitted to the structures of the 

 internal ear by means of the ossicles and structures 

 within the middle ear which convey vibrations to the 

 endolymph in the internal ear and the latter stimulate 

 the hair cells in relation with the organ of Corti. From 

 the latter end-organ they are transmitted to the 

 centres of hearing in the brain through the fibers of 

 the auditory nerve. 



Vibrations producing sound waves in the atmosphere 

 are communicated to it by means of the moving to 

 and fro of elastic bodies as tuning forks, rods, strings, 

 membranes, etc. 



