THE PHYSIOLOGY OF THE BRAIN. 615 



circular canals of the ear, which are supplied by the vestibular 

 portion of the auditory nerve; and it has, as we have seen, a 

 special cerebellar connection. An old view was that, lying 

 in three planes at right angles to one another, they served to 

 distinguish the direction of sound-waves reaching the ear ; but 

 as the direction of oscillation of the tympanic 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 sufficiently accounts for the appre- 

 ciation of notes, and such noises as are due to inharmonically 

 combined tones; while the sacculus will suffice for other 

 noises : and it is found that disease of the semicircular canals 

 does not interfere with hearing, but often causes uncertainty 

 of movements and feelings of giddiness. 



Experiment shows that cutting a semicircular canal is fol- 

 lowed 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. All 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 semicir- 

 cular canals lie. 



If, moreover, a person lie perfectly quiet with closed eyes 

 on a table which can be rotated, he is able to tell when the 

 table is turned and in which direction, and often with con- 

 siderable accuracy through what angle. If the rotation be 

 continued for a time the feeling of it is lost, and then when 

 the movement ceases there is a sense of rotation in the oppo- 

 site direction. In such case neither tactile, muscular, nor 

 visual sensations can help, and in the semicircular canals we 

 seem to have a mechanism through which rotation of the head 

 could give origin to afferent impulses, whether the head be 

 passively moved with the rest of the Body or independently 

 by its own muscles. Movements of endolymph in relation 

 to the walls of the canals may act as stimuli by causing a 

 swaying of the projecting hairs of the ampullae (Fig. 167). 

 Place a few small bits of cork in a tumbler of water, and rotate 

 the tumbler ; at first the water does not move with it ; then it 

 begins to go in the same direction, but more slowly; and, 

 finally, moves at the same angular velocity as the tumbler. 



