VIII THE MEMBRANOUS LABYRINTH 397 



he was rotating in that direction. This then is the explan- 

 ation of the giddiness which we experience under such 

 circumstances. So far we have considered the horizontal 

 semicircular canals only, but quite similar sensations may 

 be obtained by suitable stimulation of the anterior and 

 posterior canals. These are placed in vertical planes at 

 right angles to one another and are also of course at 

 right angles to the horizontal canal. As any motion 

 in space can be resolved into motions in three planes at 

 right angles, the sensations from the three semicircular 

 canals, when put together in the brain, form an adequate 

 mechanism for the judgment of any motion. 



So far we have spoken only of the sense-organs involved 

 in enabling us to co-oi'dinate our movemeftts. These 

 organs are all united to the central nervous system by 

 nerves. When we trace the vestibular branch of the 

 auditory nerve to the brain we find its course diverges 

 from that of the cochlear branch, or auditory portion 

 proper of the nerve. The vestibular branch, if followed, 

 takes us to that portion of the brain known as the cere- 

 bellum, which forms the central organ for the appreciation 

 not only of impulses from the labyrinth, but also for the 

 sensations of which we have spoken from the eyes and 

 from tlie limbs. 



Indeed, mischief in the essential portion of the cere- 

 bellum leads to giddiness and inco- ordination of move- 

 ment as certainly as derangement of the sensations 

 which we have been discussing. 



^.( AIX6 



