SENSE OF HEARING. 659 



In experimenting upon the internal ear in the lower animals, it has 

 been remarked that division or injury of the semicircular canals is fol- 

 lowed by a singular alteration in the position and movements of the 

 animal, indicating a disturbance of equilibrium. These phenomena were 

 first made known by Flourens in 1825, 1 and have been corroborated by 

 many subsequent observations, the most recent being those of Cyon, 

 Curschman, Boettcher, and Berthold, in 18t4. The results met with are 

 not explained in the same way by all experimenters, but there is little 

 discrepancy in regard to the phenomena actually presented. The opera- 

 tion of exposing the semicircular canals during life is impracticable, as 

 a general rule, in the mammalia, owing to the density of the petrous 

 bone in which they are imbedded ; but it can be done without much 

 difficulty in birds, where they are surrounded only by a loose and 

 spongy osseous tissue. The pigeon is the species which has been most 

 frequently used for this purpose. 



The most striking and constant effect produced by injury of the semi- 

 circular canals consists of abnormal oscillatory movements of the head, 

 together with an imperfect balancing of the whole body These phe- 

 nomena vary according to the particular canal which has been divided. 

 If a vertical canal be the one injured, the oscillation of the head is up- 

 ward and downward; if it be a horizontal canal, the oscillations are 

 lateral, from left to right, and vice versa. If the two corresponding 

 canals on both sides be divided, the abnormal movements are much 

 more rapid and continuous than if the injury be inflicted on one alone. 

 The animal is still capable of preserving the equilibrium of the body, so 

 long as he remains at rest ; but any attempt at movement brings on a 

 disorder of muscular action which makes walking, running, or flying- 

 difficult or impossible. The most simple interpretation of these results 

 is that the animal can no longer appreciate the direction or extent of 

 the changes in position of the head, and that the sense of equilibrium 

 is consequently impaired for movements of the body and limbs. 



The manner in which the semicircular canals, in their natural condi- 

 tion, may be regarded as contributing to the sense of equilibrium, is as 

 follows : If a glass goblet, filled with water, be turned round its vertical 

 axis, it will be seen that the water does not readily turn with it; and 

 any small objects suspended in it, or floating upon its surface, will 

 remain in nearly the same position, while the goblet revolves through 

 an entire circle. The adhesion of the fluid to the sides of the glass 

 vessel is not sufficient to communicate to it at once the circular motion 

 of the parts with which it is in contact. Consequently the water lags 

 behind the glass; and if any flat object were cemented perpendicularly 

 to the inside of the goblet, so as to turn with it, it would be subjected 

 to a backward pressure from the water, whenever the goblet were put 

 in rotation. 



1 Recherches Exp6rimentales sur les Proprit6s et les Fonctions du Systfeme 

 Nerveux, 2me Edition. Paris, 1842, pp. 452, 454. 



