730 SEMICIRCULAR CANALS. [Book hi. 



been moved. There can be no doubt that such a judgment is 

 based upon the interpretation by consciousness of afferent im- 

 pulses which are dependent on the position of the body, but 

 which are not afferent impulses belonging to sensations of touch 

 or sight, or taking part in the muscular sense. It is by help of 

 these special afferent impulses that we are aware on the one 

 hand of the position, of the relation to space, in which our body 

 may at one time happen to be, standing upright, lying down, 

 and the like, and on the other hand, of the nature and extent 

 of any change of position which our movements may bring 

 about. It is by help of these afferent impulses that we are able 

 to coordinate our movements so as to bring our body into the 

 position we desire ; and hence when these afferent impulses are 

 disordered and abnormal, the coordination of our movements, 

 the maintenance of equilibrium, is imperfect. Can we say any- 

 thing as to the exact nature and origin of these special afferent 

 impulses ? 



We learn much in this respect by studying the effects of 

 operative interference with certain parts of the internal ear. 

 When in a pigeon the horizontal membranous semicircular canal 

 is cut through, the bird is observed to be continually moving 

 its head from side to side. Injury to the bony canal alone is 

 insufficient to produce the symptoms ; the membranous canal 

 itself must be divided or injured. If one of the vertical 

 canals be cut through, the movements are up and down. 

 The peculiar movements may not be witnessed when the 

 bird is perfectly quiet, but they make their appearance when- 

 ever it is disturbed, or attempts in any way to stir. When 

 the injury is confined to one canal only or even to the 

 canals of one side of the head only, the condition after a 

 while passes away; when the canals of both sides have been 

 divided, it becomes much exaggerated, lasts much longer, and in 

 some cases is said to remain permanently. After such injuries 

 it is found that these peculiar movements of the head are asso- 

 ciated with what appears to be a great want of coordination of 

 bodily movements. If the bird be thrown into the air, it flutters 

 and falls down in a helpless and confused manner; it appears 

 to have lost the power of orderly flight. If placed in a balanced 

 position, it may remain for some time quiet, generally with its 

 head in a peculiar posture ; but directly it is disturbed, the 

 movements which it attempts to execute are irregular and fall 

 short of their purpose. It has great difficulty in picking up 

 food and in drinking ; and in general its behaviour very much 

 resembles that of a person who is exceedingly dizzy. 



It can hear perfectly well, and therefore the symptoms cannot 

 be regarded as the result of any abnormal auditory sensations, 

 such as ' a roaring ' in the ears. Besides, any such stimulation 

 of the auditory nerve as the result of the section would speedily 



