THE AFFERENT PART OF THE NERVOUS SYSTEM 81 



bound up in one of these, most massive of all purely affer- 

 ent trunks. It has been stated elsewhere that the imme- 

 diate connections made by the optic fibers are with stations 

 in the midbrain or thereabouts. From these subordinate 

 centers reflexes are doubtless caused, while, at the same 

 time, impulses mount to the cortex where the processes 

 induced become the physical basis of conscious vision. 



Hearing. The number of fibers concerned in carrying 

 the impulses on which the sense of hearing depends is 

 small compared with that of the optic conductors. It 

 has been estimated that perhaps 14,000 fibers on each 

 side are provided to subserve this function. The sound- 

 waves which have come pulsing through the air to the 

 external ear communicate their motion to a succession 

 of structures in the winding cavities of the temporal bone, 

 and ultimately to cells which can transmute this mechan- 

 ical energy to the form of nerve-impulses. The result- 

 ing currents are led to gray matter within the medulla, 

 forwarded thence to the midbrain, and finally, by a third 

 relay, to the cerebral cortex. 



The auditory nerve has a second part which is not sup- 

 posed to have to do with hearing. This division, the 

 vestibular nerve, brings to the brain impulses which have 

 been started by displacements of fluids or of solid particles 

 in the comparatively large portion of the internal ear 

 which is looked upon as an organ of equilibration. There 

 is reason to believe that when one sways or, more radi- 

 cally, when one stumbles such mechanical disturbances 

 are brought about and impulses sent to the proper centers 

 to dictate the compensating movements which are so 

 promptly observed. The service of the ear in maintain- 

 ing the balance is one which we seldom think of, and it is 

 carried on for the most part with only slight involve- 

 ment of sensation. But excessive stimulation of the in- 

 ternal ear probably has a share in developing the disagree- 

 able feeling we call vertigo. 



Taste and smell are two kinds of sensation which are 

 often blended in our experience, but the nervous paths 

 6 



