116 HUMAN PHYSIOLOGY 



apparent movement x>r parallax of these objects is a 

 familiar condition of our life. If we are placed where the 

 nearest rocks and trees are many yards away, before 

 or below us, the seeming movement of these landmarks 

 is much less than that to which we are accustomed. 

 We miss one of the clues to our own success or non-suc- 

 cess in keeping our poise. No doubt it is much easier 

 to stand at the brink of a chasm 10 feet wide than on 

 the edge of a broad abyss of the same depth. The dan- 

 ger of a fall is no greater in the second case than in the 

 first but we are greatly sustained by the visual stimulus 

 furnished by the opposing wall of the chasm. 



4. A very remarkable mechanism contributing to 

 equilibrium exists 'in the internal ear. When we men- 

 tioned the auditory nerve a short time ago we stated 

 that this nerve does not serve solely to mediate the 

 sense of hearing. One of its two divisions has this 

 function; the other is valuable because of the corrective 

 reflexes it produces when the balance is endangered. 

 This branch of the auditory nerve is called the vestibu- 

 lar. Its fibers arise in the winding cavities of the 

 temporal bone which constitute what is fitly termed the 

 labyrinth. These cavities are filled with liquid. When 

 the head is moved as one sways, or more violently 

 when one stumbles, the fluid is shaken. In it are 

 delicate filaments which appear to be connected with 

 the nerve fibers that take their departure from the 

 linings of the labyrinth. Thus we have in the internal 

 ear a receptor system which is very sensitive to me- 

 chanical disturbances and sends to the brain the im- 

 pulses which result. 



We are not usually aware of the nerve currents that 

 arrive in the central nervous system from the labyrinth. 

 When we do become cognizant of them it is to feel the 

 sensation of vertigo. It is likely that the internal ear 

 allies itself with other receptors to induce sea-sickness 

 when the body is persistently moved about in space by 

 the rolling and pitching of the vessel. When dizziness 



