588 THE HUMAN BOD T. 



they served to distinguish the direction of sound-waves reach- 

 ing the ear; but as the direction of oscillation of the tym- 

 panic ossicles is the same, no matter what that of the sound- 

 waves entering the external auditory meatus may be, such an 

 hypothesis has no foundation. The cochlea abundantly 

 accounts for the appreciation of notes, and such noises as are 

 due to inharmonically combined tones; while the vestibule 

 will suffice for other noises: and it is found that disease of the 

 semicircular canals does not interfere with hearing, but often 

 causes uncertainty in movements and feelings of giddiness. 



Experiment shows that cutting a semicircular canal is 

 followed by violent movements of the head in the plane of 

 the canal divided; the animal staggers, also, if made to 

 walk; and, if a pigeon and thrown into the air, cannot fly. 

 Ail its muscles can contract as before, but they are no 

 longer so co-ordinated as to enable the animal to maintain 

 or regain a position of equilibrium. It is like a creature 

 suffering from giddiness; and similar phenomena follow, 

 in man, electrical stimulation of the regions of the skull in 

 which the semicircular canals lie. 



Such facts suggest that the semicircular canals are organs 

 in which sensory afferent impulses, assisting in the pre- 

 servation of bodily equilibrium, arise. The unconscious 

 maintenance of the erect position depends on the excitation 

 of many co-ordinating motor centres through tactile, mus- 

 cular, visual, and aural sensations; all acting together in 

 normal combination these enable us to stand without 

 thought; but loss of any one, or its abnormal state, will 

 throw the whole mechanism out of gear. Persons who have 

 lost muscular or .tjicti]e..seiidbility:stmiA and~vvaTTTwtth diffr- 

 culty; those who have nystagmus, (jerking 'unconscious 

 movements of the eyeballs which cause the visual field to 

 seem to move in space) do the same and feel giddy; and, 

 as we have just seen, similar phenomena follow injury of 

 the semicircular canals. 



How the nerves in the latter are stimulated is not certain; 

 being filled with liquid the pressure of this on any ampulla 

 will be increased when the head is bent so as to place that 

 one below; and this may be the excitant; giving rise to 



