1010 SEMICIRCULAR CANALS [BOOK in. 



they make their appearance whenever 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 nutters 

 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 re- 

 sembles 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 die away, 

 whereas these phenomena may last for at least a very considerable 

 time. 



The movements are not occasioned by any partial paralysis, by 

 any want of power in particular muscles or group of muscles; 

 though removal of the canals of one side has been described as 

 leading to diminished muscular force on the same side of the 

 body, the mere diminution of force is insufficient to explain the 

 phenomena. Nor on the other hand are the movements due to 

 any uncontrollable impulse ; a very gentle pressure of the hand 

 suffices to stop the movements of the head, and the hand in doing 

 so experiences no strain. The assistance of a very slight support 

 enables movements otherwise impossible or most difficult, to be 

 easily executed. Thus, though when left alone the bird has great 

 difficulty in drinking or picking up corn, it will continue to drink 

 or eat with ease if its beak be plunged into water, or into a heap 

 of barley ; the slight support of the water or of the grain seems 

 sufficient to steady its movements. In the same way it can, 

 even without assistance, clean its feathers and scratch its head, 

 its beak and foot being in these operations guided by contact 

 with its own body. 



The amount of disorder thus induced differs in different birds ; 

 and some movements are more affected than others. As a general 

 rule it may be said that the more complex and intricate a move- 

 ment, the fuller and more delicate the coordination needed to 

 carry it out successfully, the more markedly is it disordered by 

 the operation ; thus after injury to the canals, while a pigeon 

 cannot fly, a goose is still able to swim. 



i 



